I left the UK as I had a job offer / promotion from the US company I worked for in London. They offered to sponsor and pay for my visa and then green card so I grabbed it with both hands.
I'd split up with a long term partner, and was making lots of crappy life choices, and thought "fuck it, why not?" I intended to come for ~18 months, and then met a yank, got married and had a kid 
I want to come back to the UK but circumstances with DS and schooling means the US is a way better bet atm