Don't know if this is just a console thing, but I'm seeing this situation more and more while solo queuing on PS4: you do 90% of the damage to a full squad of enemies but don't end up with a kill. When you walk up to a loot box and start carefully looting because you're completely out of ammo, your teammate aggressively un-mutes his mic and shouts "Hey, GTFO MY LOOT, MAN!"
First off, it's not your loot, man. We're a team. We work together to stay alive and to gear up. I was the one carefully shouting call-outs about enemy locations, what types of shields they had, and how much health they had left. All you did was show up to the party late with a Flatline and get a lucky foot shot for the kill.
Second, even if you DID do all the work and get the kill, I'm still going to loot. I won't go through and hijack everything before you have a chance to take a look, but I am going to grab a stack of light ammo or a quick syringe because I'm hurt and I need it. Hell, if I notice your health is low and you're not healing, I'll even ask you what you need and drop anything I can to help. Again, we're a team.
Where did this new unspoken rule of loot ownership come from? "My kill = my loot" seems so arbitrary – there is so much complexity to loot management based on the guns each person has, the guns you prefer to use, and what you actually need at any given moment. Why not just share?