I initially wanted to say, "don't have kids", but I actually don't see that as a huge correlate in my friends group. People I know without kids (myself included

I initially wanted to say, "don't have kids", but I actually don't see that as a huge correlate in my friends group. People I know without kids (myself included) have suffered from listlessness and lack of sense of purpose in their 30s, which leads to unhappiness. I think the bigger correlate is -- make sure you are with the right person, persons, or no-one, who are capable and willing to be open, trusting, honest, communicative, and constructive, and who have a healthy balance of selflessness and selfishness. People change, and relationships must change to weather those changes (or not) -- and that can only happen if you and your partner(s) can recognize and communicate those changes far ahead of time -- and work to adapt or move on from the relationship as appropriate.