It wasn't all bad. They were celebrities. They had a place to live, a purpose in life, they received rewards (drink and women) for their performances, and they won fame and renown. If they were a slave they could win their freedom. If they were already free, they paid off their debts and might win an honourable retirement. Victorious gladiators were often prostituted to wealthy women, and while that's pretty exploitative from a modern point of view, I wouldn't be surprised if it was viewed as a perk.

It's important to note they also weren't constantly being killed, as they were prized investments. They didn't always fight other gladiators, but were often used as performers/executioners in other sorts of 'entertainment'. Even if they did fight other gladiators a lot, they could still survive many fights and when they eventually lost, could be spared.

You might similarly ask why people become professional fighters today. Unless you make it really big the pay totally sucks, the training is extremely hard, the risk of serious injury extremely high, and you accrue injuries that make your older life harder. But people still do it in droves, and many keep doing it as long as they physically can. Things like wanting to prove masculinity, wanting fame and respect and glory, sheer aggression, a lack of other desirable options or an addiction to the lifestyle are reasons now and probably have always been reasons for people to take up combat sports, even those as dangerous as gladiatorial sport.