There are many reasons that relationships end. Some come from within, like a lack of intimacy, lost interest in one another, miscommunication, regular arguments, or unaligned goals and values.
There can also be outside factors, like health issues, family matters, cheating, meddling from other people, or unchangeable circumstances.
But deep down, the relationship ends because at least one of you does not feel happy anymore, does not see a future together, and/or your current situation is just not working.