Regulus grabs James’ shirt when he falls asleep, James finds it extremely endearing; one day he asks about it and Regulus tells him it’s a habit he developed since he was a kid because he’d fall asleep with Sirius and he’d wake up alone and scared at the house, so by doing that, he wouldn’t leave.
James, from that day on, never leaves his side when they sleep and waits until Regulus wakes up to kiss him and get up, and eventually, after years together, one day Reg stops doing it, and James cries in silence because his boyfriend finally believes and knows that he won’t wake up alone anymore.