I pick up instruments purely based on desire to play them I guess... I've never really thought about the instruments I play and why I do, I've always just kinda picked everything up and "had a go;" I've never really thought along the lines of "I want to play [this instrument] because of [this reason.]"
Piano - I've played piano since a child, purely because I grew up with a piano in the house, started tinkering around at a young age and then it grew from there...
Guitar - Because what musician hasn't picked up a guitar? Guitars are everywhere, everyone has one, they are portable and "universal" in a way.
Bass - In my late teens I just decided I wanted to give it a go, it wasn't that difficult to pick up and play having already played piano and guitar.
I would love to learn a classical stringed instrument like cello or violin, but I just don't have the time to dedicate to music to enough of a degree anymore
