For me there's two reasons - mainly, to have sex (which is a reason all its own that involves being turned on by the partner person), or to define or express the feelings I have for the one I love.
The article breaks it down to four - with sub-categories - most of which are alien to me lol:
Physical- My hormones were out of control - Usually the case with guys
- I was attracted to the person - well, duh
- It seemed like good exercise - what? cardiovasculay?
Goal-directed- I wanted to get a raise - You're in the wrong profession
- Because of a bet - so you won, or you lost?
- I wanted to change the topic of conversation - we should talk sometime...
Emotional- I realized I was in love - I like this one best
- I wanted to say 'thank you' - how do you say 'you're welcome'?
Insecurity- I didn't know how to say 'no' - so... they asked first?
- I felt obligated - can I do you a favor?