Girls initially interested in me, then unfriendly?

I think I'm an alright looking guy, I'm 19 and fairly frequently girls express interest in me. However, I have never had a girlfriend, and the same thing always happens between me and girls: 1 - The girl is really friendly towards me 2 - The girl expresses interest in me, and I reciprocate 3 - We get spend a bit more time together, or actually go out on a date 4 - She suddenly starts being unfriendly towards me, being antisocial towards me This has happened so many times and its really getting on my nerves. My interpretation is that I must be boring to talk to. Do you think this is probably the case, and if so how can I remedy this?

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