Even as a permanent resident, when entering the country it's standard procedure for Immigration to ask you why you left. Here are the last two conversations I had:
Customs and Immigration, Detroit Metro-Worth, June 2005
Immigration: What were you doing in Japan?
Moi: I'm an anthropologist, I was doing fieldwork with people with disabilities.
Immigration: Disabilities? Like me?
Moi: [Peering over counter at his legs. He isn't in a wheelchair, no braces on his legs. He is kind of overweight... I give him a quizzical look]
Immigration: [Turning very red, he mumbles] umm... I have a bad knee.. [he mumbles some more, then STAMPS my passport and hands it back] Welcome back.
The kicker, though, was the last time I came through: