After their first night of intimacy, Dean continued to call Cas down from Heaven instead of getting with random girls he met on solitary hunts. Cas knew it was wrong, and that angels shouldn't copulate with humans or get too close, but he couldn't say no to Dean. He wished Gabriel was still around; he could use his brother's guidance on the subject.
Castiel was happy, until Lisa Braeden came into the picture.
Dean said he was going on a road trip; '5 states, 5 days.' Castiel had expected Dean would call him down to earth as usual, but that wasn't what happened. Instead, Dean spent most of those five days in Lisa's loft. Cas had watched as the two met at a café in the first state Dean was supposed to be touring. They shared coffee and talked, and then Dean invited her to dinner. Even though the mere thought of Dean with anyone other than him hurt him, he couldn't stop watching until the two humans were undressing each other frantically between heated kisses.
Dean's betrayal hurt Cas, and he felt that awful feeling once more, as though his grace was being ripped right out of him. He didn't know what he was supposed to do, what it meant. Dean had said he wasn't ashamed of Cas, and they had made love, so why would the Winchester do that?
On the last night of Dean's "trip", he prayed to Castiel. 'Err, Cas? Look, we need to talk. I'm assuming you saw that, and I want to explain. Please.'
'You have exactly one minute to explain,' Cas said venomously, appearing in front of Dean.
'That's not fair!'
'Your time is running out.'
'Fine,' Dean growled. 'Look, I know you probably feel hurt right now-'
'An understatement.'
'-but I swear, I didn't mean to hurt you.'
'You said you wanted me, and then you slept with someone else,' Cas said.
'I never said we were mutually exclusive,' Dean said in protest.
Castiel grabbed the front of Dean's shirt and pulled him up so he was standing before pinning him to the wall beside him. 'You never said we weren't together either,' Cas pointed out, voice rumbling. Storm clouds gathered around the motel Dean was in, blocking sunlight from the windows. Dean looked at Cas defiantly. 'You made me believe you cared. But you didn't, did you? You didn't care about me at all.'
'Cas, stop playing the victim,' Dean sighed.
Castiel pressed Dean harder against the wall. 'Don't you dare tell me what to do, boy. How could you do that to me?'
'Well I didn't think you'd take it so seriously-'
'Are you joking?' the angel scoffed, turning his head to the side before looking back at Dean. 'I came to you all those times when I didn't need to, I did everything for you. I could have just left you.'
'Well why didn't you?'
'Because everything I do is for you, Dean!' Cas shouted. 'Everything.' His voice went quiet.
'Why? Did you fall in love with me?' Dean said, voice mocking. Cas looked at him, eyes filling with tears he tried vainly to fight. 'You did, didn't you?'
Castiel droped his arms and moved away from the other man. 'Cas, I-'
'Spare it, Dean,' Cas said with a smile on his face and tears in his eyes. He flew up to Heaven and only just managed to block out Dean's calls. He felt heartbroken, and he had no idea what he was supposed to do.
He wished Gabriel was there.