Cause of unknown depression ? distancing from social events and from opportunities to get new friendship and socializing with existing ones.

Social withdrawal: Hard to say with so little information. There can be many reasons why a person pulls back from social events and socializing. Everything is relative to what is typical and healthy for you vs now. Consider seeing a mental health professional to understand why you're withdrawing or isolating from others. I hope this helps.

DEPRESSION: Get an evaluation from a psychiatrist to help identify what depression means to you and whatever else may be bothering you. Consider recommendations for medication if any are given. Get a referral for a reputable psychotherapist to speak with. Meds and therapy combined are most optimal; no just one or the other.