Spencer hated you.
Well, how was it possible to just join the team and already be everyone's favorite, doing work on a par with him, a genius? He wasn't even sure you were older than 22. It was impossible to watch how you always got things done, how Aaron praised you even though he had never been before, how Penelope gave you the time she could spend with him, how you look so damn proper and smart.
He was sickened by the thought of you slowly taking his place and replacing him. He was sick of the way you periodically found evidence and helped make a psychological portrait of the criminal faster than he did. He was sick of your appearance, voice, manner of speaking and even being on the same plane.
New case. A serial killer, bodies dismembered and dumped in the middle of the ocean, divers found about 30 body bags. The team split up, things were going surprisingly well until you and Reid were forced to work as a team to scope out the subject and analyze the victims. His gaze avoided you, not taking his eyes off the map of the city and the casualty files. He was starting to go crazy with the irritation of your presence and attempts to talk to him. He was finished by your question about the case, which he did not even hear due to the number of thoughts that overwhelmed him.
"What the hell do you need?" he said louder than he intended, with a distinct hint of hatred in his voice.