Dean Winchester was the stereotypical bad boy. He drove the bad ass car. He listened to the bad ass music. He slept with all the hotties (that's all of them, not just the girls. So long as Dad never finds out.) He played guitar. He skipped class. He back talked (but not to Dad).
But there was a softer side to Dean. A side that loved to cuddle after sex. A side that wanted to live the 'apple pie life. A side that watched the chick flicks he claimed to hate. A side that was actually attracted more to men. A side that loved romance. A side that played piano.
Very few people saw this softer side to Dean. Sam had seen this side to Dean several times. Once, when Dad was on a hunt and he had left 14 year old Dean and 10 year old Sammy alone (when didn't he?), Dean took Sam to a bar just around the corner where there was a piano. Dean sat at that piano for almost an hour playing whatever came to mind, on several occasions he had got a round of applause.
Recently, on one of Dean's explorations of the Bunker he found a music room. In this room Dean found so many instruments from far and wide. He was instantly drawn to the guitar, which he tuned and then played until he had gotten himself back into practice. After this he only spent maybe 5 minutes a day playing it, a lot compared to what anyone else would do but just right for Dean.
After the guitar the elder brother was drawn to the piano. It was a rather standard up right piano. Obviously, it was horrendously out of tune so he meticulously tuned it. Then he tested the keys with a fast paced tune. Dean sat in front of the instrument for at least half of the day. Sam had assumed his brother had decided to play the guitar for a little longer (the room was soundproof), as he didn't know that there was a piano in the room.