Why are Relationships Inequitable?

I've written in the past about inequitable relationships, and concluded that relationships tend to be inequitable when one party has more invested, or has more to lose than the other. Purely and simply that's what creates the inequitability, but now I want to delve more deeply into the question of "why?".
The crux of the problem of inequitability lies not within another but rather within ourselves. While it is true the one we find ourselves in relationship might be a cad, or insincere, or insensitive, the problem lies inherent in each one of us, not just in our partner. We are all capable of sabotaging a good relationship with our words or actions. An example:
I was in a relationship some time ago with a "good Christian woman" I'd met on a Christian dating site. We'd dated for some time and on a number of occasions I gave her monies to help her out of financial difficulties. One day I became aware that I was missing money from my home. After several occurrences I bought a nanny cam and found that indeed she was taking monies unsolicited. Now I would have gladly given her the money, and had given her much more than she took, so why would someone intentionally put a relationship at risk over something that could have been freely received? When I asked her about it her first response was to lie, to deny, and then, fairly caught tell me she planned on putting it back. She steadfastly claimed it happened only once- the time I caught her on camera- but my suspicions of money gone multiple times prompted the nanny cam purchase. Ultimately our relationship ended- not so much from the theft, for that's what it was- but from the  lying about it. For a relationship without trust is doomed...
So why did she choose to sabotage our relationship? While her answer might be different I believe it is because we are all flawed, we are all sinners, who really don't fully understand the concept of pure love. N.T.Wright said it best when he wrote: "This is the paradox of love, in which love freely given creates a context for love to be freely returned, and so on in a cycle where complete freedom and complete union do not cancel each other out but rather celebrate each other and make one another whole". Think about that. Have you, or do you, find yourself in a relationship where you are completely free while being a total couple? It's sometimes difficult to comprehend, and yet it is fully attainable even in our flawed state. An example:
I have spent this past week down in Florida visiting my parents. Now, I've known them both all my life and observed their relationship throughout the years. It hasn't always been easy, nor has it always been perfect. But now, after fifty seven years of marriage, I see my parents both totally devoted to each other while being totally independent. They support each other, love each other unconditionally, and do for each other constantly while retaining their own identities and interests. The relationship doesn't subjugate either one of them- instead it allows them the freedom to be who they are individually even as they are recognized as a couple. That is an equitable relationship.
But it didn't just happen. It took time, and work, and dedication to each other and the relationship. It took commitment. And that's what real love must have to florish- time and commitment... Equitable relationships happen when we determine we will first give our all in the hopes that our efforts will be recognized and rewarded with our partner's return of their love. Sometimes we don't get that return and there's inequitability. But when we ultimately find it there's nothing better. We need to remain vulnerable to that possibility if we want the hope of finding it... Yet remaining vulnerable can be difficult in the face of inequity, rejection, or abuse. It is the negtive cycle to love- love rebuffed creates a context of cynicism and withdrawl, and so on until we find ourselves closed off and unavailable to real love and it's possibilities...
 Food for thought..