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a crush is mostly based on looks and a lot of other outer stuff that everyone sees.
I heard in a sermon before that the reason the thoughts are wrong are because they are baby sins. When acted upon, they grow into something larger. That's why you need to "get rid of the roots" before it grows into a large tree.
Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death.