Summarize:

Sometimes self-isolation can be a sign of a bigger problem. They may be trying to cope with abuse or trauma, alcohol and drug abuse, or mental health issues, such as depression. Your friend may not even realize that their problems are causing them to isolate themselves. Recognize when your friend's isolation isn't just them being shy or introverted so that you can support them in getting the help they need. Look for other signs that there may be a problem. For example, mood swings, changes in appetite or sleep habits, nervousness, and agitation may all be signs of a mental health or substance abuse problem. Consider whether you have noticed these changes over time or if they happened suddenly. The onset of your friend's symptoms can help you to understand it better. Ask other people close to your friend if they have any concerns. You might say, for instance, "Have you noticed anything that worries you about Daniel?" In some cases, your friend might not be self-isolating. You may not want to accept it, but your friendship might be fading. Your friend might also be voluntarily separating from society. One direct way to determine if your friend wants or needs help with their self-isolation is to ask them. You might, for example, ask your friend if they feel lonely. You might say, "Do you feel lonely or isolated very often?" Or, you could say, "Sometimes it seems like you're isolating yourself. Is this on purpose or is there something I can help with?" Some people aren't as social as other people by choice. They may prefer, and even thrive off of time alone. If your friend is an introvert, you may not need to help them at all. Unless they show other signs of a problem, respect that they enjoy time alone and appreciate and make the most of the time you spend together. Make an effort to spend time with your friend, but if they make it clear that they want or need some alone time, then respect that.
Recognize when they need help. Ask if they want help. Respect introversion.