When does, "we broke up for a reason" get outweighed by "we met for a reason?

Why is "fate" or "feelings" always so convenient? It seems like people will just take the easier answer for their situation. If someone really loves someone and something becomes an obstacle that you feel you can overcome and will make you even STRONGER because you were meant to learn from it, why do some people (the more stiff and practical ones) say "hey, you broke up for a reason. walk away", and others (the more romantic and hopeful ones) say "we were put here for a reason and we'll work through this".? People tend to leave so much up to fate and "what will be will be" when that's easier. When does actual EFFORT to overcome things become part of a relationship? If all marriages ended if u just didn't feel "into each other" that day, there would be no marriages.