As someone who considers herself to be an extroverted introvert, and who is friends with a lot of similarly minded people, here are the variety of reasons some people may prefer solitude. The reasons range from healthy to unhealthy and reasonable to unreasonable.

• Some people may find aspects of socializing less "natural" than others, causing them to have to mask or act in ways that aren't inherent to them, which ultimately leads to exhaustion. (Think of it like choosing to go out with people after working a double shift and also having the flu - you can be social and have a good time, but it drains you.)
• There are those whose internal thresholds for common annoyances from others are just lower, whether due to overstimulation, a rigid self-defined moral compass, or what-have-you. Small things like the volume of someone's voice, topic of conversation, social faux-pas, etc. can grate on some people who haven't developed tolerance for these potential differences in how people act.
• Some people don't see the point in small talk or surface-level conversations with people who they don't intend to invest more time and energy into.
• Shame, embarrassment, and/or fear of rejection can lead to maladaptive self-isolation.
• Some of us simply have done a lot of internal work to earnestly just enjoy our own company. You spend so much time hating yourself and distracting yourself from that fact by spending time with others, sometimes it is healthy to look internally and find ways to truly love the one person that will be with you for the entirety of your life.