True, it's a lot about people to me too. That's true for MMORPG games too. I no longer play MMORPGs like World of Warcraft, but I remember I was thinking:
"if this was single player, I won't play this tedious game anymore" or realizing that
"I chat more than playing". On Second Life I felt right at home: I could chat and socialize and I was free to do whatever I wished and when I wished, without the thought of character levels to make, nor the grinding that many MMORPGs have. It was "your world, your imagination" and pretty creative. And I could dress up much better!

Now I still log in often, and indeed mostly because of the people I know. At times I need some time to stay by my own, though - only managing my inventory or dressing, scripting or editing stuff, etc - but if I make an alt without any contact and I wander around alone, I can see that won't stay online in SL for long.
However, some friend quits eventually. Whether they got bored or taken away by a drama storm, but also health issues or something worse happened in real life. When your buddy list becomes more and more a list of offline people and your best SL friends stops altogether, your motivation fades. I saw that at times and I took my breaks as well.
So what keeps me coming back to SL is mostly my friends, but also to still be able to make new ones and to experience the virtual world together with them.