Dean Winchester would go to his grave denying it. He would roll his eyes and gripe at them for making googley eyes or making out in front of him. He would tell them to get a room and pretend to ignore them. The truth is, Dean would never tire of seeing his brother and his angel so happy.
He would never tire of finally seeing those dimples on his younger brother's face and the soft, fond smile on his best friend's. He would never tire of seeing hazel eyes light up when the angel walked in.
Sometimes, Dean would wonder down the hall in the late hours of the night. He would peer into Sam's room to see him curled up next to the angel, looking completely at peace in the angels arms.
Some mornings he would wake to smell bacon and fresh coffee. Walking towards the kitchen, he would hear soft giggles and humming. Stopping in the doorway to see his brother, arms wrapped around the angel's waist as he cooks breakfast, the worry lines usually between his eyebrows completely relaxed.
In all honesty, Dean loved to observe their relationship, though he would never tell anyone. He will continue to make retching noises at their loving glances, lingering touches, and chaste kisses. He'll continue to bitch and moan about them being all cheesy and sappy in front of him, yelling at them to get a room.
He'll pretend he doesn't see them lazily making out on the couch or the way Cas combs his fingers through Sam's long locks as he falls asleep during the movie they're watching. He can pretend he won't cry at their wedding if they choose to have one and he'll pretend he's never taken a candid pictures of them cuddled up on the couch.
But no, Dean Winchester will never bore of seeing his baby brother and his best friend make each other happy, but you'll never hear him say it.