why is mr.stevens plagues with regret?

I think his biggest regret is never having the strength to tell Miss. Kenton how he really feels. Miss Kenton grows increasingly frustrated by his limitations and subtly begs him to just confess his love for her so they might both live their lives to fulfillment. But Stevens cannot separate his human desire from work - and cannot find any other way of framing his own identity or sexuality without contextualizing it in work. Ultimately, this enables Miss Kenton to leave Darlington Hall and find life as her own person - as a wife and mother - while Stevens is condemned to spend the rest of his life alone at Darlington Hall, as if he's a prisoner.