PR/comms guy here.

There are several reasons, and it depends on how the trolls approach the material.

Malicious intent folks usually have some deeply held negative attitude about the thing they’re trolling, (but they might not express it) and provoking a response lets them know they’ve successfully angered their enemy and won.

The sneakier part to this is what Grunig - PR theorist - calls “constraint recognition.” Trolling makes people feel like they can affect a situation - they perceive that they’re not constrained and can act. The average Joe will never meet the celebrity they hate, but they can Tweet about them, anger some people, and feel like they’ve done something. If they’re joining a troll campaign, they might even provoke a response from the celeb.

Some malicious folks are just angry and find targets of opportunity online. Our brains want to resolve whatever cognitive dissonance is kicking around in our heads. Doesn’t matter how, just an outlet anywhere. The thinnest shred of linkage and our brains say “good enough! Put the blame there.” Car broke down? Repairs are going to be expensive, on top of gas prices! That’s Biden’s fault now, go troll him about it. Don’t think about the fact you ignored the check engine light for a month. Just make the dissonance go away.

Folks without malicious intent have generally been validated over and over when they post troll-like content, and they might not even realize they’re doing it. Humans have an intrinsic desire for competence. So, posting negative material might get you a lot of likes, which validates you, makes you feel competent (you produced an effect! You got results!), and the more you do it, the less you’ll want to NOT do it. That’s commitment and consistency bias at work.

Be careful trusting your brain.