asocial
Preferring to be alone instead of spending time with others.
Asocial describes someone who prefers being alone and doesn't seek out social interaction. An asocial person isn't unfriendly or hostile toward others; they simply feel more comfortable and energized when they're by themselves rather than in groups.
This differs from being antisocial, which describes harmful or hostile behavior toward society. Someone who's asocial might skip the school dance to read at home, prefer working alone on projects, or feel drained after spending time in crowds. They're not avoiding people out of meanness: they genuinely prefer solitude.
Many asocial people have close friends but choose them carefully and prefer small gatherings to large parties. Some brilliant scientists, artists, and inventors throughout history were asocial, finding their best ideas came when working quietly alone. Being asocial is simply a personality preference, like preferring vanilla over chocolate.
The word can also describe activities done alone. Reading is typically an asocial activity, while playing team sports is social. Neither is better: people just have different needs for how much time they want to spend with others versus by themselves.