Now we see a relationship from the other end … my dear, if your boyfriend is experiencing mood shifts — as we all do — and you don't give him adequate emotional space within which to move, you will overwhelm him with your persistent cheeriness. And he will resent you for that.
When he's down, gently ask him what he needs. Reassure him that you care for his feelings. And be patient with him (it will be an adjustment for both of you).