In this sense a crush is an immature sort of 'love' and it isn't really considered real love. For a start, it only goes one way - one person has a crush on another but the other doesn't feel the same way. Also, it tends to be something that young people get - a sort of rehearsal of feelings for when they are older. It needn't be a very romantic feeling, some youngsters develop a crush that tends to be more admiration/fascination.