Next time you see somebody playing a tank and is about to get a kill for themselves...
Let them have it.
Sure, being a DPS carry or a bruiser can easily net you that S, but when you play tanks who have nothing but CC and minimal damage, it sucks. Getting an S is very difficult as a tank, especially when no good alternate builds exist. If you're the 10/3 ADC and you're already stomping the enemy team, let that tank have the kill. Hell, he might even get an S despite dying to get his team kills because of his selflessness. Because the worst feeling ever is being unable to hit the 6 and 7 mastery levels is a struggle in itself. Trust me, I know.
So don't be a greedy little dinosaur ALL the time. Just let your tank have a kill or two if they worked hard for it. Don't be the tax collector of kills.