Often nice guys will carry a wound around not being good enough, which is in fact rooted in internalized toxic shame.
Toxic shame differs from healthy shame, in that healthy shame says, "I'm a fallible human, it's understandable that i make mistakes; however, I will take accountability and learn from them. It doesn't impact my worth or make me a bad person."
Toxic shame on the other hand says, "I did something bad, that means I am fundamentally flawed as a human; I am inherently bad and their no redemption for me."
Toxic shame arises when a child was scorned, humiliated, neglected or punished, for a certain behavior, for not being good enough some how, or for expressing themselves in a particular way. Or, perhaps they weren’t loved in the way they needed when they were in their earliest stages of development.
Developmental psychology tells us that children around 3-7 years are ego-centric in their worldview: everything is interpreted by as being directly related to them.
Thus, not being loved in the desired way is taken to be a direct reflection about them, rather than about circumstances their parents/ care-givers are dealing with. They take everything to be their fault: even how their parents aren't happy. In a way it is narcissistic to assume literally everything is about them, but this is just how a child's mind works. It does not know any better.
Both this scorning and the lack of love then leads to shame: shame around who they are, because both instances are taken as a direct attack on one self.
A child gets the sense that they are inherently wrong. And they carry that with them right the way through to adulthood. In fact such people have never not lived with a feeling of shame.
A man with internalized toxic shame does everything he can to hide his true essence, for he believes it is bad and will sabotage getting love and having his needs met. This leads to all the toxic traits related to people-pleasing, weak boundaries, inauthenticity and codependency.
Relationship psychology also tells us that we will attract partners who love us in similar ways to how we were by our primary caregivers: on a spiritual level, offering us the opportunity to heals these past wounds.
Carrying the wound of shame makes it easy for nice guys to be coerced into certain behaviours with intimate partners who can easily emotionally manipulate them by making them feel guilty or ashamed for not being good enough. This is why nice guy can often end up with partner's displaying BPD traits.
The constant suppression of self in order to appease their partner’s then creates resentment, further adding to the toxicity. This is actually what’s familiar to them. It is not uncommon for nice guys to get trapped in toxic relationships, unless he’s active doing the inner work to resolve behavior patterns and undo certain dynamics.
This is a crucial part of over-coming the nice guy syndrome, and something which can't be rushed as it requires a deep resetting of internal belief systems. Never the less you can begin the work by downloading my e-book on healing toxic shame here.