Both of those explanations are valid. Here's a little test. Can you / do you go outside? Talk to neighbors, or meet friends? Do you eat out or go to places where people are? If so, do you feel as though you're completely out of place? Do you get scared, or feel relieved once you've gotten home again? That would be anxiety disorder.

If those symptoms don't apply, then it's probably just a case of either laziness, or just fear of rejection. Either way you should work to overcome it. Social anxiety is often the result of staying away from social situations. Not many people are born with it; it comes gradually over time. Here are a couple of possible solutions:

1) Take a walk in the morning, not only will this show you what the sunlight looks like when it's not being reflected by the window, it will also give you a chance to exercise, and relax, and acclimate to the sound of the outdoors. If it's too hot, then try doing it earlier, or later. Around six or seven am it's usually fairly cool, and there aren't too many people around.

2) Once this or a similar habit has been formed the next step is to call family members or old friends. Chat about this or that, anything, but make sure it lasts about five to ten minutes. (Do some research before hand, and get the names of kids or spouses). This will improve relation-building skills, and even if you do embarrass yourself, they probably thought you were odd anyway, so it won't hurt anything.

3) Next make a few crank calls. This will build confidence in phone skills, as well as socializing, and talking to people skills.

4) The hard step is this one: Meet the neighbors. (If you know the neighbors, then invite them to dinner, if they aren’t that type of neighbor, skip them and go to the next). Again, this will give you socializing skills, but in case of embarrassment will not hurt you severely.

Once these or similar steps are taken, (and repeated if necessary), the final step comes.

5) Call that person (or people) you were dying too, but couldn't quite.

This will be the final stage in acclimating you to the world. Enjoy, and don't revert.