On a standard view of desire, necessarily, one has a desire for A if and only if one has a tendency to pursue A. But even if this is true, it does not answer the question of what a desire is. One could identify the desire for A with the tendency to pursue A. But that would be mistaken, because the desire explains the tendency, while the tendency does not explain itself.
Perhaps, then, the desire is not the tendency, but the desire is defined as the immediate cause of the tendency, whatever that immediate cause might be. This suggestion is preferable to simply identifying the desire with the tendency. An interesting consequence of the standard view of desire conjoined with this identification is that necessarily every tendency to pursue A has a cause. This need not, however, be taken to commit us to a general Principle of Sufficient Reason (PSR). For it might well be that the notion of a behavioral tendency entails the existence of a cause, and indeed of a unitary one. If George on one occasion pursued A for one cause, on another he pursued A for another cause, and so on, that would not add up to a desire for A, and, if this is not to be a counterexample to the standard view, it would also not add up to a tendency. A tendency requires a unitary cause.
If this is right, then the standard view very neatly fits with a definition of a desire as the immediate cause of the tendency. But if we think about it, it's easy to come up with counterexamples. What if George has a tendency to pursue A because whenever the question comes up, Dr. Black zaps his brain in such a way that George pursues A. There is thus a pattern of pursuit of A, and this pattern has a unitary cause, namely Dr. Black. But Dr. Black is not identical with any of George's desires. Moreover, in a case like that, I think, we would not want to say that George has a desire for A.
Alright, so we need to modify the standard view, or at least to clarify the notion of a "tendency". Only internally-rooted tendencies count. But that's not good enough. For suppose that George's liver has a weird mutation such that it grew the neuro-zapper that Dr. Black was using, and the liver regularly zaps George's brain so that George ends up pursuing A. Now the tendency to pursue A is internally rooted in George. But it's not internally rooted in the right place. It's supposed to be internally rooted in George's mind. But that, too, wouldn't do. Suppose for simplicity (and contrary to fact—but I think the argument is very suggestive even without the false assumption) that George's mind is identical to his brain, and that his olfactory center grew the same neuro-zapper. Now the tendency is internally rooted in George's mind, indeed, but in the wrong part of the mind. Moreover, easy thought experiments show that not only must the tendency be rooted in the right place in George's mind to qualify as entailing the presence of a desire, but it must be rooted in the right way. In what place and in what way? Surely only one answer is possible: the tendency must rooted in one of George's desires, and the rooting must be of the right sort for desire-based motivation. The "right sort" condition will ensure that the tendency must be rooted in George's desire for A.
So, our standard account now says that one has a desire for A if and only if one has a tendency to pursue A that is caused in the right way by a desire for A. This isn't very helpful as an account of what it is to have a desire, is it? But it's not completely vacuous. The definition entails that a desire for A causes a tendency to pursue A.