Always the same

Why is it that it's always the same people who are fun and spontaneous? Like when you try to organize a going-out it's always the same ones shouting "YES!" and "of course" and "fun", whereas the others have various degrees of excuses. Acceptable is "I'm on vacation in another country", "I'm already invited out" and possibly "I'm sick". Not acceptable is "I have to wash this evening", "I'm watching tv", "I have to go to the gym" or worst of them all "I have to work". Where is the fun in your lives people?