I must say that I believe fetishes are not genetic, and are actually linked to subconscious associations from one's past.
However, whatever the reason, I didn't used to be comfortable with it either, but you like what you like man. There are plenty of other fetishes out there that are much more harmful than tickling. Not to say that if someone is into those that that's bad. A little pain here and there can be fun too 8C P
But I like making people laugh. If you feel the same, then it makes sense that you like doing the same in your sex life. Just tickling is making someone laugh through affection/ physical contact, where as you would normally do it verbally.