...which motives are predominant to learn and play an instrument and to present these abilities including own singing to the public. Which are the true reasons behind this?

The answer seems to change with every decade. When I was 16 it was because I wanted to play guitar, and be popular generally. When I was in my 20s it was to meet girls. Later, in my 30s it was because my straight job was boring me silly. In my 40s I realized I didn't know how to do anything else. I am now in my 50s, and I am avowedly interested in corrupting the youth of America by teaching any and all those interested to play the 5 string banjo. What my 60s will bring Heaven only knows!

Jim K