1) A was an experiment created to be used as a weapon. He was made to be stronger and more resistant than any human and scientists constantly tested just how resilient he was during his childhood. This caused A to become terrified of the scientists and he'd avoid talking or even listening to them. Because of this he was deemed a failure and when B a new scientist arrived they decided to pass A off to him making him B's experiment. B decideds to take the time to build trust with A and show him not everyone is a bad person.
2)A is a scientist who was attempting to turn animals into humans. He succeeds with B and is beyond thrilled spending lots of time with him to teach him everything. This causes B to become quite close with him and when B notices A becoming close with another scientist he begins to grow jealous.
3) make your own
(Let me know if you want to do smut :3)