People say that love spells backfire for many reasons. A popular one is the caster not really realising the True effect it will have. The movie "Fatal Attraction" (Not a kids movie) perfectly shows what *can* happen when love becomes obsessive. The vast majority of love spells are designed to Cause an attraction within a person, to you. Love cannot be created or faked, love is far too strong and pure of a force to simply force another to feel it.
Instead, obsession occurs which is far easier to compell.
Other reasons they tend to fail is the person casting isn't capable of effecting the person they want to effect, the person has greater strength of will or stronger energy. Its also possible the caster could be inexperianced and is simply devoid of the proper energy to cast the spell properly and effectivly.
Finally, many people refuse to cast love spells or refuse to discuss them because they violate the individual will. Some people find the idea distasteful as a principle and others feel that if you resort to such spells, the true issue is internal not external.
It isn't ever wise to use magic with the sort of desperation that a heartsick person tends to have.