The fact that Mike tried to kill himself the same place Will's body was found— after the police investigation suggested Will had died falling over that same cliff into the waters below.
This was after his body was allegedly found and pulled from the quarry, after Mike saw this with his own eyes, bicycling home in a hurry to cry in his mother's arms.
Mike was giving up on the slim hope he had that Will was still alive and able to be saved, that the body wasn't his, and that he would be found with the aid of Eleven who, by this point, had presumably disappeared for good.
So, when he gets the push he needs, he cares so little about his life as it stands that he is willing to die on impulse to protect his friend. He's already lost what matters most to him: the friend he cares about most is, by all estimations, dead, and even the slim chance he had of saving him is vanquished by El leaving.
And he does not just try— If El had not spontaneously appeared and saved him, he would have died. He didn't just move to the edge and think about it. He stepped off.
He made the final decision in his mind to die, with the simple wish of helping one friend who was still there in his last monents, and he must have known he inevitably would.
"So, if your friend jumps off a cliff, you're gonna jump too?"
He was ready to join him.
"I don't know how to live without you" fits his relationship with Will far better.
