From verse 48: لَا يَمَسُّهُمْ فِيهَا نَصَبٌ وَمَا هُم مِّنْهَا بِمُخْرَجِينَ (No weariness shall touch them therein nor will they be expelled from there), we learn about two features of Jannah: (1) No one shall ever feel tired or weak. This is contrary to what happens in our mortal world where hard work naturally results in fatigue. In fact, even a state of total rest and fun would sometimes tire a person out, no matter how enjoyable are things one is occupied with. (2) The other feature of Jannah we discover is that the blessings once received there shall be eternal. These blessings shall never diminish nor shall anyone be expelled from there. In Surah Sad, it was said:إِنَّ هَـٰذَا لَرِزْقُنَا مَا لَهُ مِن نَّفَادٍ (This is sustenance provided by Us which is never to exhaust - 38:54). And in the present verse, it was said: وَمَا هُم مِّنْهَا بِمُخْرَجِينَ (nor will they be expelled from there) that is, they shall never be deprived of the state of bliss bestowed upon them. This is contrary to the affairs of the mortal world where the recipient of the best of conveniences lives under the apprehension that the Over of the facilities may become displeased with him one of those days and expel him out.
As for the third probability, that the blessings of Jannah may remain everlasting, nor one is expelled from there, but one may himself get bored by living there and wish to go out, the Qur’ an has demolished this probability in one sentence of Surah Al-Kahf with the following words: لَا يَبْغُونَ عَنْهَا حِوَلًا (That is, these people too would never seek to return from there - 18:108).