Because of this, Margaret felt that she had no reason to have a lack of trust in Mike, and eventually gave him a large sum of money and shot another man in order to save his life. Basically, the movie is saying that sexual relations can sometimes lead to irrational trust between two people. The movie also shows what can happen as a result of a break in this trust. When Margaret finds out that everything between her and Mike, as well as everything they took part into together, was all a sham to con money out of her, she ends up nearly losing her mind and ends up chasing Mike down and shooting him to death. This is reflected in real life often through how angrily people react when one of their friends has stolen one of their things or when a man or woman catches their spouse cheating.
Relationships are also perhaps the main theme of the movie Blue Velvet.
Rather than focus specifically on how one certain type of relationship affects the people involved, Blue Velvet more compares different types of relationships, ranging from the normal to the absurd. The first relationship seen in the movie is that between Jeffrey and Sandy. This is essentially the normal relationship of the movie. Both Jeffrey and Sandy are clean cut kids of the same age, class, etc, and was built on a strong foundation of friendship. This relationship seems to basically be a standard for which the other relationships in the movie are to be compared.