I honestly never used to get a lot of Rage Quitters online. In fact it was very, very rare for people to Rage Quit against me; however I noticed that after the Youtube Feature was added I began to find a lot more people SDing mid-match than usual. There's never any lag in my matches a lot of the time and if there is there's very little of it, so I can't understand why they would give up so easily.
I noticed it typically happens when I take a stock off of them and have a huge lead (Only having between 30% to 50% on my first stock while they're on their second). I can understand if you don't want to stick around to get your butt handed to someone but it would still be nice to finish the match. You may never know, someone can easily screw up or you can make a great comeback.
Rage Quitting like that only shows me you're a poor sport and makes me lose respect for you as an opponent.
Aside from 1V1s though, I also get it often in 2V2s when me and a friend play. In fact just yesterday we encountered these two players who got KOed only twice I believe before proceeding to SD constantly. At the end of the match they were -30 behind because they SD'd so much during it. Again, just finish the match. It'll earn you some serious respect from me for at least trying. Fight till the end!