This is based more on the US series of Shameless than the UK series. Set after 2x08.
Mickey doesn't need Ian. He's doing just fine without him. He doesn't need to be fucked so hard that he can only feel aches instead of his body. He doesn't need to have Gallagher smile at him every day, knowingly. He doesn't need Ian.
But, fuck it. He wants Ian. He's not doing fine without him, who is he trying to kid. He wants to be fucked so hard that he can only feel aches. He wants Ian to smile knowingly at him every day. He wants Ian.
Which is why, one Saturday afternoon, he pulls Ian from one of the aisles in Kash's, where he is stacking shelves, and drags him into the storeroom. It's also why Mickey kisses Ian, hard, smashing their mouths together. Ian is surprised, but willingly reciprocates and, when they break apart, Mickey whispers harshly, "Fuck it, Gallagher. Want you. Need you." And Ian understands.
Because Ian is a drug and, as unwillingly as Mickey is to admit it, he's addicted.