Close interpersonal relationships aren't generally easy. If you yourself happen to be a font of inspiration and charity in all things, you have a pretty good chance at making things smooth. If you yourself happen to have a selfish, entitled, judgmental attitude (and almost surely are unaware of it), you have pretty much no chance at making things smooth. If in between, well, in between--and it depends on how your imperfections mesh or clash with theirs.

Life is full of joys and regrets. The grass on the other side of the fence might look greener due to disposition rather than actual lushness. So we can try to measure the greenness of the grass.

Studies generally indicate that having a partner is where the greener grass is, if you can find a stable partner: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8654342/

(It also shows that the grass is only somewhat greener. It's not guaranteed disaster vs. assured bliss. Beta's only 0.1-0.3 in the categorical regression, which is roughly equivalent to "kinda important but totally doesn't override every other consideration".)

Is this because people who are satisfied with life generally find it easier to be a partner, so it's all a selection effect? Maybe. But it might also be very fundamental to how our instincts pull us.

So be careful when talking up the challenges of building a life together with someone else. Yes, there are challenges, but that just means that there are tradeoffs, and that it's important to select the right partner.