Just depends on my mood and other factors.
If I'm flying solo (which is about 90% of the time) on business, I don't mind having friendly conversation with people (strangers) sitting next to me if they're not annoying and or creepy. If they do meet those two elements, I'll usually do a quick hi and put on my headphones.
Overall, I like a good balance of conversation and some privacy to where I can listen to my MP3 player and focus on getting work done on my laptop if need be.
If I'm flying leisure with friends or family, I'll talk to them the entire flight. No problem there.