You want to know the real reason why relationships fail?
Think of it this way,
If you're lonely, another person will not solve your loneliness.
The problem is that you're alone. The problem is that you don't feel whole and complete within yourself. And this is why so many relationships fail. Because people want someone to fill a void that they feel within themselves.
If you get into a relationship with someone for that reason, you didn't solve your inner problem, you just involved an innocent person in your inner problem. You are looking for an external solution to an internal problem.
Your relationship will have problems because your problems are what caused you to seek that relationship in the first place.
A good relationship starts with you bringing your whole self to that relationship, the best version of yourself to that relationship. And that's how the relationship succeeds.
No one can make you happy.
Happiness is just a state of mind. And it's something that you have to bring to the relationship. If you're looking for someone to solve your loneliness, you will probably be alone forever.