the old adage 'be careful what you wish for' is very true.
while the cost of a spell isn't something as extreme as loosing and arm and a leg, there is a cost. just by casting the spell you need to use energy and focus. you also need to put forth the effort for the spell to manifest [cast a job spell, pass out applications] however, your gain will be anothers loss [cast a job spell to pass the interview means other people loose the job]
love spells are another can of worms entirely. people argue the free will of the person you're casting over being effected, while the spell does have some influence, you need to be careful in a number of ways. wording for example, lets say you cast a spell in which you say 'i want him to want me so much he can't stand it' this could result in him becoming possessive and clingie. or say you cast a spell over a guy who has a girlfriend an they break up, don't you think the ex is now heartbroken? [this gets even worse when they conclude he was cast over and a magickal tug of war over a guy ensues.]
plus, if you believe in karma or the Threefold Law, you could see punishment through other means. say you break up a couple, don't be surprised if he leaves you for another woman. casting one too many spells can sometimes result in the universe/gods teaching you a lesson [if you cast too many improperly you could also start feeling ill because spells require energy, using the energy your body needs can make you feel weak. this happens too often and you can get sick]
this is why so many of us on here suggest you think about casting for some time before actually proceeding. in the moment, ruled by emotion, you might decide 'no! they wronged me! they must be punished!' and curse somebody. two weeks later however, you would be a lot calmer, and realise the big curse worthy pain wasn't as bad as once thought. too often people cast without thinking it through properly [in emergencys, it's fine to cast right now without thinking, but most people fall into 'i love him, and he loves me, he just doesn't know it yet' or 'they wronged me in some small way which is huge to me, they must pay']