Your question is pretty self-answering. The main reason for resentment was that Dumbledore disclosed important things from Harry, and Harry thought that the reason for disclosing these things was that Dumbledore:

• did not trust Harry
• did not think Harry was capable or strong enough to handle these things
• was too self-centered
• was simply ignoring Harry because of other too-important matters.

Your question is pretty self-answering. The main reason for resentment was that Dumbledore kept important things from Harry for the six previous years, and Harry thought that the reason for not disclosing these things was that Dumbledore:

• did not trust Harry
• did not think Harry was capable or strong enough to handle these things
• was too self-centered
• was simply ignoring Harry because of other too-important matters.