In many languages, including English, the most important verbs are irregular. Examples include:
The same applies (roughly) to many other languages I know about (Dutch, German, French, Spanish, Swedish) and presumably to many other languages too.
Is there any reason why these everyday verbs tend to be irregular?
These everyday verbs have another feature: they have been in the English language for a long time. The oldest verbs were ones that were borrowed from other languages, or have come into English from Old English, before the patterns of regular conjugation were formed. Consider to go and its past tense went. The following is from http://www.etymonline.com/index.php?term=go.
Old English gan "to go, advance, depart; happen; conquer; observe," from West Germanic *gai-/*gæ- (cf. Old Saxon, Old Frisian gan, Middle Dutch gaen, Dutch gaan, Old High German gan, German gehen), from PIE *ghe- "to release, let go" (cf. Sanskrit jihite "goes away," Greek kikhano "I reach, meet with"), but there is not general agreement on cognates.
The Old English past tense was eode, of uncertain origin but evidently once a different word (perhaps connected to Gothic iddja); it was replaced 1400s by went, formerly past tense of wenden "to direct one's way" (see wend). In northern England and Scotland, however, eode tended to be replaced by gaed, a construction based on go. In modern English, only be and go take their past tenses from entirely different verbs.
So how did went become the past tense for go?
The etymology indicates that it probably came from the past tense of wenden.
The verb to get has a similar Old English pedigree.
Contrast these ancient verbs with newly-coined, modern verbs. The modern verbs will always be regular. For example, You gross me out. / You grossed me out. / You are grossing me out.
Or imagine that you are coining a new verb, for example, to smurf, meaning to pelt with a blue doll. You would undoubtedly come up with a regular construction, namely I smurf proudly. / I smurfed when I was younger. / Your smurfing is becoming tiresome.
Contrast the Old English verbs with verbs that have been repurposed. To fly is an Old English verb and it is correct to say, I flew out of Atlanta. But watch what happens when you use the verb to fly in baseball. When a batter hits a ball to the outfield and it is caught, the ball is called a fly ball and the play is called a fly out. The next time he comes to bat, the announcer will usually say he flied out. This modern usage of an old verb follows a regular construction: He flies out. / He flied out.
Please take a look at this list of about six hundred regular English verbs. Contrast with the much shorter list of irregular verbs. Although both lists have everyday verbs, it seems that the largest factors are when and through which language they entered Modern English.
I can identify at least one mechanism that contributes to this phenomenon, at least theoretically. A language is continually under pressure from two directions: contraction and analogy.
Contraction happens when we pronounce words quickly or sloppily and some sounds are dropped or fused or mixed up. The latter is not strictly contraction, but you know what I mean. In general I would say paradigms become more irregular when contraction occurs, such as can't versus cannot or guv'ner versus governor.
Analogy is what happens when we change certain forms of a word to more closely resemble a pattern related to another word. Normally, analogy moves in the direction of regularity: a word will acquire a form that more closely resembles a dominant pattern. So, for example, children may say breaked instead of broke.
The theory goes that people are more likely to slur over very frequent words, because they are so familiar that they are easily recognised and quickly pronounced. Less frequent words are supposed to be less susceptible to contraction. On the other hand, people are more likely to temporarily forget that an uncommon word has an irregular form, such as the plural antennae: it is more likely for it to become antennas exclusively than for men to become mans. So contraction and analogy work together, in a way, to produce less regular frequent words and more regular infrequent words. Of course there are other factors, too, which may work against or alongside these.
This is purely my own opinion, but it seems to me that for centuries English has been gradually regularising existing verb forms. And when new verbs are introduced, they're almost always regularly conjugated.
It also seems fairly obvious to me that irregular conjugations learnt at an early age will tend to be preserved. For example, in later life, I'm sure I read learned more often than learnt in contexts like the previous sentence, but that hasn't significantly affected my usage.
It's important to note that I personally don't feel threatened by the possibility that my use of English might be considered non-standard or uneducated. Particularly in respect of something like learnt/learned, where I'm in the company so many other BrE speakers (both living and dead).
Children soon notice how regular verbs work, so they naturally generate "incorrect" past tense forms such as eated, taked, rided. But no-one wants their children to sound ignorant/uneducated, so usually parents assiduously correct any such mistakes.
Obviously with less common verbs, firstly the child is less likely to use them, and secondly there's a greater chance that the parents aren't familiar with the "correct" (but irregular) forms. So they get corrected less often, and with the passage of generations, the irregular forms can gradually die out.
Thus it seems only natural that the most tenaciously-preserved irregular forms will mostly be very common verbs learnt in early childhood. Old habits die hard, and all that.
One answer is that it might be easier to learn verbs where distinct meanings correspond to distinct form, e.g. be in present and was in the past. The frequent use of these verbs might then act to preserve this association of form and meaning. Thus it's not surprising that several of the verbs you quote have different roots in different tenses or persons in many European languages.
Another deviation from 'regular' morphology would be the verbs that retain a specific form of conjugation that is no longer productive, e.g. the systematic pattern of vowels in the same root (ablaut): sing-sang-sung, a residual pattern that you find both in Germanic and Baltic languages.
I'm not sure if I would call this pattern irregular though -- maybe their continued survival highlights that the coexistence of several different patterns in the same language is not really a problem but rather a very natural phenomenon.
Wikipedia address your question at http://en.wikipedia.org/wiki/English_irregular_verbs There are only four references, but I suspect the first one will provide an authoritative source for you. In general, verbs that tend to be irregular in English are "borrowed" from OE or other languages and have retained elements of their original conjugations. Notice that these verbs are important in the sense that they usually describe common, everyday human activities. I do not know where the regular conjugation we now use comes from. Our brains have evolved to seek patterns, hence children tend to over-regularize; I suspect adults would do the same thing were it not for the rules of grammar.
