In general, a lot of people 'act nice' when they want something. When they don't get what they want or no longer interested, they can turn nasty. Sometimes this is a simple process, sometimes they can do an act for a while. It seems that in your case, there is some kind of communication issue if you truly don't know, but sometimes the missing reasons can be there, but we don't take those in account or agree with them, hence the confusion.

Though, I would probably not go out of my way to use this place as a pick-up site, as it is not the best for that type of thing. Not that I can suggest any alternatives because most of the others I know of are terrible too. I personally find online dating to be a meat market, especially with 'hook-up app' culture such as Tinder.

All I would recommend is ensuring you love yourself first. This is reminding yourself that you are a great person with lots to offer, and you deserve to be treated well. If people don't have the decency to be considerate, then think: "Thank Goodness, they ejected themselves from my life."

With that in mind, all the best in the future.