Concerning team-killing and the like

I'm new to R6: Siege. I've been playing for roughly a month now. I apologize in advance as I assume this question has been asked time and time again, but as my title states, why does this happen so much? Every two or three rounds, people start the team killing right off the bat. It sometimes happens for long stretches as well, say, two to four games in a row. This applies to both the multiplayer and terrorist hunt game modes. It's really frustrating and is kind of turning me away from the game despite how fun it is otherwise. I'm not asking anyone here to get into the heads of other players who engage in this, but is there a reason why this happens so damn often? It's pretty depressing because I really do like this game. I wish there were a real answer besides, "Those dudes are just assholes." Guess I need to find a dedicated squad to play with.