I studied physics because I liked it and the way it describes our world. I then went for a PhD and slowly realized that physics (except a few areas) is basically dead. Particle physics for instance deal with completely outwordly stuff, akin to planning a family trip to Jupiter. Sure, it may happen someday but there are more present things.

I got my PhD and left academia (because of this and petty politics around photocopier paper costs) and the only part I really miss is the teaching and the bright minds.