I really hate this mentality in general. The notion that you must be perfectly happy with yourself to be in a relationship.
Here's what I'm gonna say- the depression itself is rarely the issue, it's how you deal with it, and more specifically, how you expect your partner to deal with it. If you expect your partner to solve all your issues, if you get mad at them when they don't know the right thing to say or do, then that will grow into a problem. However, if you discuss your issues with them calmly, or just focus on the positive in your relationship to help you cope, then it shouldn't be a major issues, unless your partner is an *******. I've never dated anyone with severe depression, but with my friendships, it's pretty clear that this is where the issue is from. I've posted about this around the forum before, so I'll try and be brief and not turn into the guy who preaches the same thing on every thread:
There's a huge difference between: "Everything is messed up, help me, why does no one care about me" and "Hey, I'm feeling a bit down, can I talk to you about it?"