Sadly for Samson, something that might have been one happy part of his life turned out to be the absolute worst. disaster. ever. (Fun fact: Milton was drawing on personal experience here. He was one of the first people to make a Biblical argument for divorce.) Both of Samson's marriages to Philistine women end with betrayal, raising questions not only about marital fidelity, but also about fidelity to one's nation or people. In fact, it's a kind of uncommon and interesting aspect of Milton's writings that he so often focuses not simply on the passions of romantic love and sex, a typical topic of literature, but equally on the complexities of marriage as a personal and social contract. In Samson Agonistes, marriage is as much a political act as a private act.
Dalila is obviously the unfaithful one here. You don't turn your husband into the authorities, plain and simple.
Samson's misunderstanding about what a good marriage should be like is the number one source of his problems.