Paradise is very high, and ascending 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 al-Bukhaari and Muslim from Abu Hurayrah (ra) the Messenger of Allah (saw) said: “Hell has been veiled with desires, and Paradise has been veiled with [surrounded by] hardships.”
An-Nasaa’ee, at-Tirmidhi, and Abu Dawud report from Abu Hurayrah (ra) that the Messenger of Allah (saw) said, “When Allah created Paradise, He told Jibreel, “Go 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 said, “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”“. [Jaami al-Usool, 10/520, 8068]
An-Nawawi commented on the first hadith:
“This is an example of the beautiful, eloquent and concise speech which the Prophet (saw) 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 including 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]