It's moral to act spontaneously as long as your reason tells you that doing so is consistent with your values and your life.
For instance, suppose you have a spontaneous desire to go out to eat at a nice restaurant tonight, instead of cooking at home like you normally do. All else being equal, acting on this spontaneous desire is just pursuing your values.
On the other hand, consider the same situation, except that as soon as you have the desire, some issues occur to you. Can I really afford this, on my budget? Do I have the time to go out tonight, given that I have a project due tomorrow? Will I be able to stay on my diet?
Now, you might still go out—but morality demands that you not evade these issues. You have to decide whether they are valid objections, and make a conscious, deliberate decision based on your values. Not that this has to take a long time—sometimes a moment's thought is all that is required to deal with concerns like these. But to push them out of your mind without thinking is to evade.
Evading concerns to pursue a desire turns it into a whim, in Ayn Rand's usage of that term: "A 'whim' is a desire experienced by a person who does not know and does not care to discover its cause." (“The Objectivist Ethics,” as quoted in the Ayn Rand Lexicon entry on Whims/Whim-Worship).
Morality does not demand that you pursue a value only after long and serious consideration. Acting spontaneously or impulsively can be fine. Morality demands only that you use your mind to guide your choices and actions—that if you pursue a value based on a desire, you do so by permission of your reason. The whim-worshipper puts emotion in charge and allows it to override reason.
answered Feb 07 '11 at 02:52
The first clue is that one actually try to rationally evaluate one's desire.
If you don't, that is, if you just do what you want to, without considering any possible reasons not to, then you are following a whim.
Of course, it's possible to rationalize a whim, in which case you are pretending to consider reasons not to follow it, and making up reasons to follow it.
In the end, this question is like asking how do I know if I'm being honest. You know because you know what being honest consists of: not faking it.
Following your emotion (or whim) means pretending that you've considered everything important in your final judgment of whether to do something.
No, you shouldn't arbitrarily torture yourself looking for things which might possibly indicate that you should not follow your desire. But any reasonable concern must be considered before making one's choice.
So, be spontaneous except when there's a good reason not to be.