I started playing osu!standard because an online friend told me it had helped his aim in CS:GO improve (this turned out to not help me at all
)
I then discovered that people played with tablets, and lucky for me both my sister who studies graphic design and my brother had drawing tablets, so I began playing osu a lot more.
After a while I got bored of standard and wanted to try something different, so I went full time ctb.
Osu is indeed pretty cool because it gives off a very familiar vibe, with the playerbase not being enormous and the fact that peppy cares a lot about his game.
Personally also a big fan of the fact that your competing directly on a ranking leaderboard and the fact that you're the sole person who can influence your own improval (compared to team games like League Of Legends)