Their lives aren't miserable per se, they just have mental problems and think that moving to Thailand can solve them. Unfortunately, they tend to bring their problems with them and the novelty wears off after a while. Then, they generally have to troll other people in order to chase that feeling of specialness that they got when they first arrived.
Many kind of have their self-esteem attached to their perception of Thailand. They view Thailand and Thai people as being superior, and they are then able to feel superior by extension through being in Thailand and claiming to be "more Thai" than other forefingers.
In terms of behaviour, this means they end up hating other foreigners whilst claiming that all Thais are perfect. They'll even go as far as saying that if a foreigner does something negative, but a Thai does the same, the foreigner is bad for doing it, while it is fine for the Thai to do the same.
Total delusion.
The real kicker, is that they will claim to be happy, when their behaviour suggest the complete opposite.