It has nothing to do with not protecting the "fed" person... you misread me.
Go back and watch one of your own replays.
There was more than likely a time where X diver/assassin was on Y team mate and you were off diving the enemies instead of taking 2 seconds to stun that other guy so your team can fight together instead of flipping a coin at who wins the teamfight.
It's something as simple as pressing Locket 5 seconds earlier when you're closer to them so they get the shield too, or holding onto your ult to toss it onto their most fed member when they dive for your mid in the middle of a fight.
In the end you should win more games too, since you are watching for and predicting what the enemies will do, and saving something to hit them with when they do come in.