"The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one." -George Bernard Shaw
As Shaw points out in that nice snippet, happiness isn't terribly reliable. There's no reason to believe the universe is geared towards our emotional well being.
In the more literal sense, one of the dangerous aspects of this woo is when it refers to physically feeling better: Quackery. Many alties talk about how they feel better after using some alleged treatment for a condition. This is one thing I find to be anti-holistic about "alternative medicine": All the various ways we can feel bad can be considered symptoms of the problem. Quacks rely on the placebo effect so that their victims can rationalize ways to think less of their symptoms and pain: During natural variation in the symptoms, they can rationalize high points as the quackery of the time being working, and bad periods as flare-ups, perturbations from the treatment, or other such things. It's focus on the symptoms, not the causes.
In fundie circles, "knowing" that they'll get a pleasant afterlife gives comfort, but it usually comes with something nasty for non-believers, regardless of how moral they are. Fairly selfish in my opinion. Also quite nasty is that these preparations for the afterlife can come in the way of enjoying life or doing the right thing. Don't pick a religion because it makes you feel better. Pick one because it's got evidence for it. If something is wrong with the cosmos, knowing the truth about it is the first step to solving it.
For more generic woo, it often gives people a false feeling of being special, like being psychic, indigo, or whatever's hot this month. I'm usually worried these people will underachieve, feeling entitled in their ivory tower, rather than seeking out their real talents or learning something worthwhile.
You don't need nonsense to be a good person or feel better about yourself. You just need to find what you're good at.