I always found this a funny one - I tried (unsuccessfully I might add) to learn guitar first, a few years ago, and I just never really got on with it. Naturally I preferred to play with my fingers and I just couldn't use a pick for the life of me. Having a 'gap' between what I could feel really threw me... so when I decided to give bass a go a few years later naturally i played with my fingers first. I'd always loved the way that a bassist's fingers glided from the higher to the lower strings with such an effortless, butter-smooth motion (That's something I still haven't quite nailed either)
And again when it came to a pick, I just couldn't seem to get it, so I avoided using one. I think the first song that I learned that forced me to use a pick was Ace of Spades, and that was the one I always practiced on, and that's what let me nail playing fast with a pick... I think it was a confidence thing more than anything, as is often the case. I think it's down to the song though, what sounds and feels right