Saying how you feel is easy until someone answers. The speaker starts with a familiar idea, that words come faster than actions, and then leans on the science line about every action having a reaction. It sounds almost like an excuse, a way of stepping back from something painful before getting to it.
The turn comes when the speaker admits "I love being here" and gets a cold answer: stay if you want, but don't expect me to be there. The poem suggests the speaker was being open and hopeful, and the reply shut that down. Nobody in the poem explained that this reaction was possible.
The last two lines are short and bare. "The reaction I didn't want. / The action I must've deserved." The mood is hurt, but also self-blaming. The speaker seems to take the rejection as a punishment, which makes the ending quietly sad rather than angry.