1. Society is over-sexualized. This is mostly because corporations, musicians, directors have realized that an appeal to sex is the best way to sell literally any product. As a result members of society are encouraged to believe that sex is the most important aspect of life. Naturally, those that don't have sexual relations feel like they are missing out on the most important part of life.
  2. In reality incels are upset about the legitimate discrimination that men face. But since opposing this discrimination openly basically ends up with you being cast out from society anyway they have no motivation to confront the legitimate discrimination they face in a reasonable way and feel they might as well just go after women.
  3. Dating apps. In more natural social environments men's romantic success is dictated both by how well they are accepted by women as well as how well they are accepted by men. As a result a man who feels socially and therefore romantically rejected is not only resentful of women but also somewhat of men. On the other hand, dating apps put the sole responsibility for rejection in the opposite gender's hands. To make matters worse the nature of dating apps is that decisions are made primarily on superficial characteristics so men feel that women themselves are superficial. Finally, dating apps disproportionally show attractive profiles which raises both the expectations of men and women and results in fewer satisfied relationships and more incels.