Guys, why do you dissappear?

I have been actively speaking to a guy for some time now. He has openly shown that he was interested in me and having a relationship with me. I wanted to take things slow and to get to know one another because I wasn't sure about dating him. Then finally when I was starting to show signs that I was interested in something more he disappeared. How do you explain this behavior?
Guys, why do you dissappear?
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