Dr Hendrix’s conclusion is because it increases the commitment, and so inevitably brings on a power-struggle. Differences that seemed minor before marriage, suddenly seem terrible once our destinies are bound together. Those differences will exist for any couple, perhaps even more so the more they love each other.
You can choose to look at the power struggle as a bad thing, avoid the arguments, and maybe build a little coolness and distance into the relationship. Maybe instead you enjoy the passion of a good flaming battle of the wills. But you can also use your marriage as the opportunity to grow more closely together, and through that become more complete and alive as individuals too.