Buddha said that desire is the root of all suffering. But what is the root of desire?

Ginger. I know, I know, it's a rhizome.

or

Knowledge. We covet what we don't have, and if we don't know we don't have it, we can't desire it. Ignorance is bliss.

Misunderstanding the nature of self.

The discrimination into self/other which occurs when one has a naive relationship to thoughts is at the core of desire: it sets up the situation "I am incomplete without X. X is 'out there', I am 'over here', and I must obtain X to resolve the void I feel inside".

The root of desire is ignorance/wrong views. From ignorance/wrong views comes desire, lust, greed, attachment, ego, etc. Technically the Buddha never said that desire was the root of all suffering, although it is true that desire and attachment certainly do us harm and cause lower or unpleasant rebirths according to the Buddha.

If we were not ignorant and didnt have wrong views, then we would not be desirous or have an ego in the first place and we wouldn't be under the false assumption that we all have an independent existence separate from everything else, creating the idea of a "self", the idea of "me" or "mine".

The root of all UNNECESSARY suffering is ignorance/wrong views. The other kinds of unavoidable suffering such as physical pain or the loss of a loved one are just things we have to accept and make peace with.

Love and fear.

All fear is built around on perceived loss of love, acceptance, validity, self respect (joy, safety, security - pick any descriptor of feeling loved).

Fear makes us horde, defend, deny others, acquire, compare, judge, as well as making us feel insecure, unprepared, needy.

That's the basis for desire. All desires have a 'because'.

Ego & Ignorance...

Ignorance that placating your desire will fulfill your ego.

Ignorance of the absolute reality of inter-dependent origination.

Attachment to an outcome. I believe that Buddhists think all is good and to be one with the universe is to be detached from the outcome. Desire comes from attachment to an outcome..you root for "your" team..your desire is that they win. If you don't care who wins, if you can see the good in any outcome, your life flows smoothly in sync with nature..you don't fight it! :)

The root of desire is in the fact that, according to Buddha, we don't really understand the real predicament that we are in. Living is suffering. Everything we do, no matter how pleasing, must end. This is the suffering. The Buddha feels that because of this suffering, we must escape it. Now here comes the desire: desire is actually a misunderstanding of what is good. Most understand it to be things like friends, money, success, marriage but according to him, it is not the good. What is good is to escape this world completely into nothingless-ness. Karma comes into play because if one still has desire for this world and its joy, then their karma isnt pure or detached enough to pass away. They( their essence or "bundle of perceptions") is recycled/reincarnated into a new fetus. Interpreting Buddha, desire is explained thus.

human nature.

I studied it in Thai language, so my word might be wrong when explaining in english.

Tanha is the root of desire.
I know that in English like wikipedia they say tanha=desire, but from what I practice, desire mean longing, wanting for sth, its characteristic is you will feel burn, uncomfortable inside. Like a man seeing a beautiful lady naked. while Tanha mean "the taste": what you like, what you don't like. It's like this if the same man sees an ugly girl naked, would he feels the same as case one, maybe even want to look away. Why? You can say because she isn't beautiful. Then I might ask you what is beautiful, one thing that one things is nice can be ugly in other's eyes. It's your TANHA, because you don't like it, then you don't want it. But be careful, there is still desire here, the desire to LOOK AWAY. So if you have no taste, feel nothing at all to everything, there is no desire.

And the root of tanha is because we don't know the truth. That everything is a void, no me no you. If there is no you, no me, who will have the taste? The objective and subjective is no more, just void.

The root of "desire" in the context of this question of it is attachment to an outcome of the expression of a conceived desire in the belief that you or your life will somehow be better for it, thus creating an attachment which imprisons us and causes suffering. Freedom from attachment allows us to experience without suffering so that the expression of a desire comes free of worry about whether or not we have it which in turn means we are infinitely patient and then able to enjoy whatever abundance (material or otherwise) that may come to us as a result. This is the law of the universe and by it we may know and expand the knowing without the suffering.

Buddha is very wise. And to keep in line with a wise man, one must look at the word suffering. There is an old way of suffering that is sweet and intoxicating. It is called delayed gratification. To suffer, to wait, to anticipate, to plan. The desire is the root of all anticipation and planning. To obtain that what has been adored and worshipped from afar for many a year, then to obtain it's ripe sweetness is a reward for one's suffering, delayed gratification.