I don't understand. Why do I do this?

Eh, could be a gift, could be a curse. Maybe u'd make a good actor. Also it shows you're open minded and can see things from different points of view... putting yourself in someone else's shoes in other words. In another sense it could mean you're not comfortable enough with who you are to where you're trying to wear other peoples' personalities to mask your own. Maybe you really just like role play.