There are some reports that crush may have stemmed from the fact that it is somewhat similar to the word mash which means head over heels during the earlier times. There is also another term called ‘spoony’ which means to be amorous towards someone. Spooning right now means being close to someone, right?
Knowing these things, it will not be hard to think about why the word crush already means this way right now. When you have a crush on someone this means that you are physically attracted to the person. Having a crush on someone is perfectly normal as long as you are not hurting other people in the process.