I have Steam set to launch on demand (i.e., not start on startup). Sometimes, after I launch a game like Torchlight 2, which also launches Steam, it somehow fails to connect and dumps itself into offline mode. Well, one option is for me to check that it's online every time it's launched. This method, however, is tedious and prone to human forgetfulness; there is also the fact it sometimes goes offline randomly during a session. It also seems to me that once it goes offline (against my will), it doesn't come back online automatically? (could be wrong here). Anyway, in my case, I've had to open up Steam, go to File->Go Online.. to switch it back.
The reason I want to do this is for games like Torchlight 2, you can't earn achievements when Steam is offline. I found this out the hard way.