Usually pure lust. I HAVE to have it because I think it is so beautiful, unusual, bright, soft colored, big, small, eye catching, cheap, you name it! lol
The biggest mistake is looking at orchids online. Oh my, then I have to have at least 3 of them!
I guess the biggest couple of determining factors are whether I have money at the moment, my husband agrees its ok for me to buy one (he's a soft tough! lol) and if I can get it home. Sometimes I do stop to think where I'll put it, but usually not until after I buy one. It's very surprising that I only have 30+ of them!