The only thing I'll add to the discussion is to make sure you get a breed that fit's your temperament and oh my God...make sure it's well trained.
My girlfriend and I have a Boxer, and it's a great dog and I love her to death, but she has A LOT of energy and the breed needs CONSTANT attention. This okay for awhile, but my girlfriend and I are introverts so sometimes, we just want to chill. There are several times where I just want to be close to my girlfriend and we'll start to snuggle in the bed, on the couch, etc. and the dog will whimper and whine until we pay attention to it. Or we'll hug for a bit in the kitchen, and the dog will whimper and whine. Or we'll let her get in bed, and instead of just chilling at the foot of it or on the sides where there's plenty of space, she'll try to get in between me and my girlfriend.
Most of the times we just laugh it off, but there's a few times where I have to restrain myself from picking her up and tossing her across the room.
The dog is already 3 years old, and we've broken a few bad habits (we've gotten her to sleep on her bed when I'm there), but she's still got quite a few left and I doubt we'll be able to change them so we just have to put up with it.
So make sure you get a dog that fits your lifestyle, i.e. don't get a high-energy, high-maintenance dog if you just want to chill all the time, and PLEASE get them trained EARLY.