1/2 In canon, I think Dean would top when he and Cas first got together. Partially cuz he’s not ready to be that vulnerable, and partially cuz he’s excited to show Cas all about sex so he wants to take the lead. Then, as he and Cas got to really trust each other physically, he would want to bottom because Dean LOVES to be taken care of. And he doesn’t like to admit it, but he really likes to not be in control because he’s so very tired of worrying about being in control of everything.

2/2 For once, he just wants to be vulnerable and let someone else take the reins. Cas on the other hand has constantly been out of control, particularly when it comes to Dean’s safety. So Can generally likes to top, because he loves to have control in this one aspect of his life – to have control of Dean’s pleasure and be able to take care of him. That’s my headcanon anyway ✌

I can see it. And I can see Dean being super freaking attentive when he tops for Cas because how could he not be, right? Like, I just think Dean’s a really giving lover and giving his partner pleasure gives him pleasure. So having Cas underneath him, a little blown away by all the new sensations would be just as fun for Dean as it is for Cas. And of course, Dean would be just as attentive to Dean when the tables are turned. 

I just like the idea of give and take with these two. Like, they really enjoy getting lost in each other and providing each other with what they need, whatever that may be.