The reason is that Steven simply doesn't want to talk or think about it, as he is sick of the past continuing to pull him and his friends backwards instead of forwards ("I just want to fix it"). He didn't have to have a tantrum, though, but that's just his mounting frustrations throughout the entire series being let out through an increasing spike of angry bursts.
It's also hormones and puberty. Everyone acts badly at this time of life, so it's understandable.