Paradise is very high, and ascending to lofty places takes a great deal of effort.
The way to Paradise is filled with things that go against human wishes and inclinations. This needs strong determination and willpower. In a hadith narrated by Bukhari and Muslim from Abu Hurayrah, the Messenger of Allah, sallallahu alaihi wa sallam, said: "Hell has been veiled with desires, and Paradise has been veiled with hardships."
The version recorded by Muslim says "surrounded" instead of "veiled." [Jami al-Usul, 10/521, no. 8069]
An-Nisa’i, Tirmidhi and Abu Dawud report from Abu Hurayrah that the Messenger of Allah, sallallahu `alaihi wa sallam said:
"When Allah created Paradise, He told Jibril, ‘Go and look at it.’ So he went and looked at it, then he came back and said: ‘By Your Glory, no one will hear of it but he will enter it." So He surrounded it with hardships and siad, ‘Go and look at it.’ So he went and looked at it, then came back and said: ‘By Your Glory, I fear that no one will enter it." [Jami al-usul, 10/520, no. 8068. Tirmidhi said: It is a gharib sahih hasan hadith]
An-Nawawi commented on the first hadith:
"This is an example of the beautiful, eloquent and concise speech of which the Prophet, sallallahu `alaihi wa sallam, was capable, in which he gives a beautiful analogy. It means that nothing will help you to reach Paradise except going through hardships, and nothing will lead you to Hell but whims and desires. Both are veiled as described, and whoever tears down the veil will reach what was hidden behind it. The veil of Paradise is torn down by going through hardships, and the veil of Hell is torn down by giving in to whims and desires. Hardship includes striving consistently and patiently in worship, restraining one’s anger, forgiving, being patient, giving in charity, being kind to those who mistreat you, resisting physical desires, etc." [Sharh an-Nawawi `ala Muslim, 17/165]