So true. My day is so totally different when I exchange a good morning with my suite mates. I have a friend who says "good morning" when she greets me regardless of what time of day it is. A good morning always seems to get me started on the right foot.
Actually, the same thing is true of a goodnight. Saying good night gives closure to the time you've spent with other people throughout the day, but isn't as final as good-bye. Good night has the feeling of "I'll see you in the morning to wish you good morning".
This morning as I was leaving for class, one of my floor mates stuck his head out of his door and said good morning to me. When I leave my roommate in the room every morning (or when she leaves me), we say "I'll see you later."
Conclusion: Good Morning and Good Night build a feeling of community and that's why they make the day better.
Alternate note: I do not enjoy when people just say "Morning". That takes the friendly "I wish you joy today" and turns it into a frank statement of fact about what time it is. Very few people can get away with it. Some people, yes. And I'm certainly guilty of dropping the "good" from my morning greetings. But I don't really like it.
Interesting to note that as I've been writing this (I'm outside Chik-Fil-A) three people have wished me good morning. Hooray for college community! and for it being warm enough to sit outside!