[zwieback]

It's easier to reply "sorry, busy right now" in response to "hi". Once they say what they want I feel more obliged to make time for them.

[ktal]

Can't really determine if I actually have time for something before I know what it is. And unfortunately, I don't have the luxury of just ignoring things I don't want to deal with.

[hinkley]

There's a huge space to occupy between 'hi' and launching into your life story before you know if the other person is even reading.