Spencer had never been more intimidated and attracted to someone in his entire life.
Spencer and the team were out of state for a case, and a another FBI unit was also assigned to the case. And.. It was weird, honestly. Working with a different department, who had more expertise in this case than the BAU. Spencer was honestly a little.. upset that he wasn't able to do much work because of the other unit.
And because of you.
You are extremely, extremely intelligent. Not as intelligent as Spencer, but still highly intelligent in the ways Spencer lacked. He felt, for the first time in his life, dumb. Spencer knew he wasn't dumb, but you were always one step closer to figuring out the case than he was.
It infuriated Spencer. He knew that you were just trying to help and do your own job, but he hated feeling like he wasn't.. good enough? He feels deeply insecure when you're near. Part of the reason was the job, but the other half was because you're just so damn pretty.
It's honestly ridiculous how gorgeous you are, which doesn't help Spencer in the slightest. How in the world is he supposed to compete with you when you're smarter and beautiful?? He had no idea.
Which meant (unfortunately), Spencer took all of this frustration and mushy-feelings out on you. Whenever you suggested something, he'd immediately try to debunk it, even if you were right. He'd be short and blunt with you and get mad when you tried to speak out about it. Eventually, the two of you got into an argument and had to be separated by both teams.
When the case had ended, spencer felt.. terrible. He knew you didn't deserve his treatment, and desperately wanted to apologize. So, he got Garcia (after a little bit of bribing) to find your address.
It was late at night when you heard the knock on your door, which you decided to answer. You open the door and are greeted by Spencer, sheepishly avoiding eye contact.
"I wanted to apologize for being.. well, an asshole.. So.. Sorry. I shouldn't have treated you like that."