Why guys ignore me when they know I like them?

It's been recurring in my love life.
My first boyfriend ( we had been online friends for sometime) and he when i expressed my feelings (indirectly) he picked up on my feelings and started ignoring, we got in touch later but after 2 years i hurt him and he left and never came back.
My second boyfriend, it started by messing around, I started ignoring him as i told him that i shall fall in love with him if we continue seeing each other. He convinced me we can continue seeing each other, there was another girl in the picture (it was his sex friend, I was getting suspicious and we fought) however when we fought i was so passionate and asked for forgiveness because i was feeling guilty. I think he understood that i truly like him and he started ignoring me. after 2 years he contacted me back and i never replied.
My third crush, he seemed interested but was bringing another girl in our conversation. I knew something was wrong and tried to sort out the puzzle. I understood that he had not move on from his first break up and helped him. I was also interested in him and he would give me his attention but was not respecting me, i kept investing my time and money in him (giving luxury gift). I then discovered that he is seeing/ helping one of his friends (a girl), i threw a tantrum and ultimately when I told him that i love him with a sincere heart, he started ignoring completely.
why does all my partner and potential partner does this? what am i doing wrong? what do i need to change?
Why guys ignore me when they know I like them?
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