They say once you reach Nirvana you remove craving. However, don't we need to crave the state of Nirvana in order to reach it? Doesn't that contradict everything an enlightened person who reached Nirvana stands for? Because in order to continue this state, meditation and other things need to be constantly practiced which is a sort of craving in itself.
You do not need to crave Nirvana to reach it.
Nirvana is present in every moment. The belief that you need craving to observe Nirvana is contradictory because it is incorrect.
Since it is always present, there is no prerequisite to observe it. This is how spontaneous enlightenment is possible.
Because it is present in every moment, there is no maintaining that needs to be done.
The reason the Buddha created his teachings was to combat craving. Craving is what distracts us from observing Nirvana in every moment. That's why he teaches non-attachment. It is only through uninterpreted experience that the illusory separate self and Nirvana can be seen clearly.
How do you reach Nirvana when you are craving Nirvana?
You don't. You have to stop all craving in order to "reach" "Nirvana", including the craving to reach Nirvana. However, it makes sense to first stop the coarse instances of craving, then the less coarse ones, and finally stop craving for Nirvana.
At least this is how it is described in the books. In real life, craving for Nirvana stops somewhere along the way (75% in or something like that) but one may still have residual subtle cravings for things to be different then they actually are at any given moment.
For example, I no longer crave Nirvana, but I occasionally crave to have a different job, one that would involve teaching Dharma full-time. Or sometimes I crave the lifestyle of people that are fully integrated with society and have friends and family etc. These are examples of cravings even a relatively advanced Buddhist practitioner might have.
In the Gradual approach to Enlightenment, all of these cravings must be abandoned in order for Nirvana to fully manifest. So even though you "got it", you still keep working to iron out these smaller wrinkles. This is called "Sudden Enlightenment / Gradual Cultivation". To some this sounds fine but to others it smells of an infinite progression.
In the Sudden approach to Enlightenment, one just accepts these occasional cravings as something inevitable due to the Impermanence and no longer craves to stop having them. This of course only makes sense in the overall context of Wisdom Freed From Reification. In other words, when your practical realization of Emptiness reaches a level when you stop seeking a silver bullet, when you see things as they are and solve problems on case-by-case basis -- then you are at peace even when you are not, occasionally. This is so-called "non-abiding Nirvana".
To "crave" and to "will" are 2 completely different things. If you're not willing to go to the park, you'll never be at the park:
"In that case, brahman, let me question you on this matter. Answer as you see fit. What do you think: Didn't you first have desire, thinking, 'I'll go to the park,' and then when you reached the park, wasn't that particular desire allayed?"
"Didn't you first have persistence, thinking, 'I'll go to the park,' and then when you reached the park, wasn't that particular persistence allayed?"
"Didn't you first have the intent, thinking, 'I'll go to the park,' and then when you reached the park, wasn't that particular intent allayed?"
"Didn't you first have [an act of] discrimination, thinking, 'I'll go to the park,' and then when you reached the park, wasn't that particular act of discrimination allayed?"
"So it is with an arahant whose mental effluents are ended, who has reached fulfillment, done the task, laid down the burden, attained the true goal, totally destroyed the fetter of becoming, and who is released through right gnosis. Whatever desire he first had for the attainment of arahantship, on attaining arahantship that particular desire is allayed. Whatever persistence he first had for the attainment of arahantship, on attaining arahantship that particular persistence is allayed. Whatever intent he first had for the attainment of arahantship, on attaining arahantship that particular intent is allayed. Whatever discrimination he first had for the attainment of arahantship, on attaining arahantship that particular discrimination is allayed. ~~ SN 51.15 ~~
I think this question is answered in the Brahmana Sutta (SN 51.15). The Brahman asks --
If that's so, Master Ananda, then it's an endless path, and not one with an end, for it's impossible that one could abandon desire by means of desire.
This doctrine reminds me a little of the Bikkhuni Sutta too, by the way.
1 "Craving" is maybe like a thief in a dark room -- wants to steal something, have something, anything, doesn't even know what!