In my opinion, the main reason why some people find it difficult to relax is chronic stress. People who are constantly dealing with stress in their lives may have a hard time relaxing because their bodies are in a state of constant 'fight or flight'. This can make it difficult for them to calm down and let their guard down. Similarly, people who are naturally inclined to be more mentally active may have a hard time turning off their thoughts and relaxing because they are prone to worrying, ruminating, or overthinking. Anxiety and depression can also make it difficult to relax as they can make it hard for people to switch off their thoughts and feel at ease. Past traumatic experiences can also make it difficult for people to relax because they are constantly on guard and re-living those experiences in their mind.