Anyone can be affected by depression. There doesn't have to be a traumatic or heart-wrenching experience behind it. Being alone is enough of a good reason to feel the way you do.

I am happy that you have taken the time to think about how your behavior affects your relationships. Whenever I feel guilty for being neglectful of my relationship with someone, I make myself listen to them attentively and allow them to talk about what you want to talk about. As a life long introvert, I've learned that showing enthusiasm to hear what others want to say will rope in a lot of friendships. (This is assuming that these are people who would listen to you if you chose to talk about yourself...)

If your depression is not too deeply entrenched, maybe a few sessions with a therapists will be enough to identify what causes you to the feel the way you do and what you can do about it. If you do have troubling depression issues, then you'd need to be seeing a therapist anyway. So, I think it's a good idea!