because, lets face it, the ex has already had a head start.
meaning they've been together much longer than you.
he knows more about her or him than you.
that is enough to make you feel insecure.
he also can tell if you still love the guy.
and some guys are just seloso.
it boils down to trust.
does he trust you enough?
if you trust your girl, there is nothing to be insecure about.