Why do people have relationships?

I'm guessing you live in a cave.

I'm guessing you live in a cave.

I'm thinking the same thing. Why would one not want one?

Other than sex, children, and legal stuff(if married), of course.

Whatever have the Romans ever done for us, apart from a stable government, aquaducts, roads, wine and a million other wares we now can get hold of??

Or something like that..

I'm sure a relationship makes happy many humans. It might be another reason.

Fear of being alone and financial reasons are probably near the top. The OP didn't stipulate "healthy relationships".

Other than sex, children, and legal stuff(if married), of course.

Various cultures have had their own theories on the origin of marriage. One example may lie in a man's need for assurance as to paternity of his children. He might therefore be willing to pay a bride price or provide for a woman in exchange for exclusive sexual access.[21] Legitimacy is the consequence of this transaction rather than its motivation. In Comanche society, married women work harder, lose sexual freedom, and do not seem to obtain any benefit from marriage.[22] But nubile women are a source of jealousy and strife in the tribe, so they are given little choice other than to get married. "In almost all societies, access to women is institutionalized in some way so as to moderate the intensity of this competition."[23] In English common law, a marriage was a voluntary contract by a man and a woman, where by agreement they choose to become husband and wife.[24] Edvard Westermarck proposed that "the institution of marriage has probably developed out of a primeval habit".[25]

In some cases, male will kill the children to start new relantionship)

Well that was helpful.

I didn't say it wasn't true. It's just not helpful to a discussion about why humans get involved in relationships to tell us that male lions will kill cubs so the female lion can mate again. Are they going to buy a condo together? No? Then? How is this helpful information?