We replaced our kitchen sink about two months ago, and realized there was a leak on the cold water line where the hose from the faucet meets the copper pipe. After retightening the nut, everything was dry and fine for about a month until last night when the basement floor under the kitchen was soaked. I looked at the fitting again and noticed it was loose (not loose enough to tighten by hand, but could turn it 3-4 times with a wrench until snug). Question is, how is it possible for this fitting to continually loosen itself? Is it something I need to continually monitor or should I assume it's just a bad fitting and replace?
The hose from the faucet looks like one of these, right? (Doesn't matter which one exactly, they're almost all made the same way).
If so, the connector has a washer in it, and it sounds like the washer in yours has gone bad. Tightening it too much can over compress the washer and cause it to fail either right away, or prematurely. (So it's probably not that the connection is getting looser, but that the washer is breaking up, allowing you to turn it more).
All you'll need to remove the hose is a wrench (though getting up near the faucet can be tricky). Bring it to your local hardware store/home center and you should be able to pick up a replacement for under $10.