I got a friend who keeps talking about a colleague. My friend is pretty disappointed by the other guy's behaviour and keeps on harping about it day in day out. Is there one word/concept which sums up this behaviour of my friend?
Sounds like infatuation to me.
It normally has the connotation that you're in love with the target, but it can be used for other obsessive passionate emotions as well (eg, loathing or frustration in this case), as the infatuated party is being foolish in their obsession.
This useful word was perhaps originally coined by Helen Fielding in Bridget Jones: The Edge of Reason. It would be particularly apt if your friend is actually attempting to conceal a secret interest in his/her colleague.
I think your friend is inclined to gossip: ( from TFD)
information about the behavior and personal lives of other people
a chatty talk
Since you are hinting at a pathological attitude I may suggest:
- is an anxiety disorder (OCD) characterized by uncontrollable, unwanted thoughts and repetitive, ritualized behaviors you feel compelled to perform. If you have OCD, you probably recognize that your obsessive thoughts and compulsive behaviors are irrational – but even so, you feel unable to resist them and break free.
The person could also just be upset and is therefore venting; he/she may be also be vindictive, or aggressive, depending on content and tone, or may seek opinions about what happened, validation etc. Emotions affects behavior and speech. You describe a voluble, prolix, loquacious colleague, depending. The nouns can also be used. The choice of a word to describe the behavior will often vary with perception of, and level of empathy towards, the other person.
I incline to the view that your friend is obsessional concerning the other person's behaviour (http://www.thefreedictionary.com/obsessional). Obsessive Compulsive Disorder is considered a mental health condition and of a rather more serious order (http://www.nhs.uk/conditions/Obsessive-compulsive-disorder/Pages/Introduction.aspx).