I'd suppose it could be a (kinky) way to humiliate you, to affirm his superiority, or to forget/compensate his feeling of inferiority. Or then it could be something he has seen in a pron-movie, just playing copycat.
Most guys like doing this for a little while until they realize that it doesn't feel as good as it looks in porn. It's no big deal. If I were you I'd let him do it just so he isn't left wondering. Keep your eyes closed and kleenex nearby and it'll be fine.
exactly what the 1st guy said. that's pretty gross. I don't know why guys like it so much. I knew a girl who it supposedly got in her eye and her eye swelled up and got all red. she looked like quazzy moto almost for a week. if it sounds gross to you, don't do it, its a little disrespectful