Some people say it is outright wrong to use "I and my friend" or "I and my X". Their reasoning is it should be "My friend and I love the sun" is right because when separated "My friend loves the sun" and "I love the sun" both makes sense. But if I use "I and my friend" love the sun, they get split as "I love the sun" and "My friend love the sun". The latter is wrong. Others say the sentence is right grammatically but not polite to use "I" before. So who is right?
The only reason why "I and my friend" is discouraged is because it's considered discourteous to put yourself first.
When you say "My friend and I love the sun" the subject is plural, and then the plural form of "love" is required. When you say "My friend loves the sun" you must use the 3rd person singular form of the verb -- "loves".