Thor fears Steve. Not for his anger or skill; but his sadness.
Thor is a god; his lifespan will last thousands of Midgardian years. Yet he has chosen to tether himself to this mortal realm, these mortal friends.
Even if they survive their dangerous lives, they will age and die. A few years, decades at most, they will all be gone. And he will have to live the rest of his lifetime with only their memory.
Thor fears Steve, because in him he sees the wounds such loss leaves behind.
...
(It is not his mortal friends he should fear for…)
AN: Then Thor :Ragnarok happened...