When you agree with someone, you share the same opinion, feeling, or purpose, like when you agree on the movie you will go see on Saturday afternoon.
If we were living in 12th century France, we might use the phrase a gré to describe things that are favorable or go well for us. This positive outcome is still part of the verb agree, because if people agree with you, your plan or idea is the one that will be used. In grammar, subjects and verbs agree when they are both singular or both plural. For example, "everyone is here" agrees but "everyone are here" does not.