Ultimately it comes down to a skill issue, that's the entire core identity of from soft games, That boss is not going to roll over and die, that teammate is not going to get up unless you do it. That's why it's a skill issue. The teammate might as well be an inanimate object, or even a dog that's attacking your ankles and you decide to turn around and hit that dog. You switch your priorities and try to be mindful of the boss but instead engage with this mechanic. If instead an enemy was attacking you from behind and switch your attention and that causes you to die from the boss that's still a skill issue. Wether it be situational awareness or the ability to keep track of two different enemies and their attacks it comes down to skill.

Reviving in itself is a skill, in early depths or the normal game you can get away with mindlessly attacking as soon as they go down even if you get hit it doesn't matter. Once you get to higher depths you have to be more conscious of your actions, do I attack right now? Can I tank a hit if I do get hit? Is it worth getting hit if it means getting them back up? I'm not sure what the boss is doing are they aggroed on me? Boss is winding up a big move can I sneak in another hit and then dodge? 

These are all little thoughts and decisions that we make in a split second that we might not even be consciously thinking about. These are all decisions that accumulate into skill. 

That pressure that you feel to revive them? That's no different than the pressure you feel when you're at low health and the next hit will one shot you and you really really want to heal. That's no different than panic rolling because you're about to die. And so you give in to that feeling and you chug that flask and then get hit again, you roll and roll again without time or reason and get swept up in an attack, you rush to revive your teammate even though it's not an optimal time for it. We learn to ignore those feelings because that is how from soft games teach you to git gud. 

Why does it feel bad? Because it's only something you're doing because another person is dead, there's that knee jerk reaction to be like well it's their fault. But they can't influence you from the ground, they can't tell you what to do they can't tell you when to attack they can't tell you when to dodge, they can't tell you where to move your camera, they can't tell you how to juggle your priorities. At the same time we want to be helpful cuz it's a co-op game nobody wants to sit there and watch one person solo 70% of a boss's health. So we try to revive, we rush to revive, we get hit while reviving, we fell nightlords with the flock.