But if he would not have desired for knowing the truth of life how could he be able to know the truth of life,it means to desire is must.
Thanks for asking me.
The "desire" the Buddha was referring to was about sensual desires. Sensual desires refer to desires of the 5 senses. These sensual desires are unquenchable. It is the very nature of physical senses. Take for example, hunger. Once the hunger is satisfied with intake of food; the process of depletion starts, and after some time one feels hungry again. Another aspect of sense desire is the stronger feeling of craving. One's craving to satisfy one's desire is itself a catalyst to crave for more. When this craving goes unchecked, one becomes crazy! So the option is opened to anyone who follows the Buddha's teachings: to continue feeding these desires with more craving, or to come to one's senses to reduce this crazy cycle of on-going "madness".
As for "desire" to know the truth of life or striving for enlightenment, it is a very different concept. Here we are talking about one's commitment to reduce one's greed, hatred and delusion until complete eradication. This is not a life-long process, but countless life-processes. "Desire" is not an appropriate word to describe this journey towards enlightenment. “Aspiration” will be a more appropriate word. Of course anyone can still argue until the cows come home that it is still "desire". So it is up to you to decide. No big deal if you still insist that it is "desire". The important thing is that I have given you my opinion. See, this is the beauty of Buddhism.