I've been severely depressed, and although I can't fully relate to your situation I feel I could maybe offer you some of the "why" part of it.
When I felt depressed I shunned those around me. It was more than just closing off, I genuinely felt negative towards them. My family, my friends, my girlfriend...I found reasons to dislike them and to push them away. The one thing I really hated, was no one sat down and just listened to me, and agreed with me. Those few I did try to talk to kept offering me advice on how to 'fix' myself, and how to 'get over it'.
Do you and your wife go out much? If you don't maybe try and do that? It's possible that there is no fun at the moment for you two, and with her depression it's likely she's projecting her feelings onto the relationship.
If she says she still loves you, I believe there is hope. It might be difficult, but there is still hope. If you do ever talk to her about anything, do it at her pace, don't push her, but don't slow her down if she lets it all out. Although it's hard, you will have to be the easiest person to be around, someone she can relax with.
I hope this helps in some way, and good luck.
