Most Helpful Guy
The first thing to hold in your mind is online sites attract people who have commitment and rejection issues. These are people who aren't getting dates which means they are not necessarily completely stable under all conditions. Just in case, I am among that group of Internet daters.
So you find someone you like and maybe they like you and everything looks good. Something may trigger a change in thought or their situation changes. You aren't commited so they don't feel an obligation. They may also not say anything because they don't want to hurt your feelings.
I have on occasion asked for some feedback. Basically I simply say I just want to find out what I am doing right and wrong so I can have a better chance in the future. People have responded to that.