Most Helpful Guy
as a guy there is a natural inclination to be a provider. it isn't about being better than women, it is about doing our best for them. we, in many ways, show our love and prove our love by working hard so we have more money to spend on the things she wants, and needs.
when she makes more than you do, for some guys, they feel emasculated because she has no reason to need, want, or even expect, you to do these things for her. when a guy is used to showing how much he cares by providing for her, he can feel without purpose, or at least like he has no real way to express how much he cares.
if you're not the kind of guy who thinks like this, its no big deal. if you talk to her about it, and work things out to deal with your awkward emotions on the issue (how i would deal with it) everything can work out great. But for a lot of guys, they feel like they have nothing to offer her, and may even resent her (unfairly) as if she took from him his ability to show how he cared.