well, not all cases that you reject an offer is because the company is not good or bad. may be you got other better option/offer in mind.

but not all things go smoothly,for instance,you go for 2 interviews,company A is offering better job and salary,but long distance and long hour.company B offer lower salary but shorter hour and short distance. both of them give you some time to think about it. ok,you go home and think for one week and decide to go for A.before that you call B saying you reject their offer coz of bla bla bla this A company is better money. then when you call this A,they saying that they already hired one better candidate. you are angry but there's nothing you can do about it.

that's why when you reject one's offer dont give them excuse like, i have accepted other company because they pay me higher/Better benefits than you can offer. think carefully before you want to reject. and if you really want to reject think carefully what to say so you dont hurt/Insult them,becoz you never know you might consider them again.