So, in summation, their first merit as pointed out in this verse Ali 'Imran 3:170 ⧉ is their distinct perennial life; the second is their being well-provided from Allah and the third (فَرِحِينَ بِمَا آتَاهُمُ اللَّـهُ ) is that they shall always be happy with Allah's blessings and grace. The fourth (وَيَسْتَبْشِرُونَ بِالَّذِينَ لَمْ يَلْحَقُوا بِهِم) merit is the good news given to them about their relatives and circle of friends they left behind in the mortal world that they too shall deserve the same blessings and ranks with their Lord if they remain good in deeds and are active in Jihad (Ali 'Imran 3: 170⧉ 171⧉).
Al-Suddi says that a شھید shahld is informed beforehand when one of his close friend or relative is about to die. The news that a certain person was coming to them makes them as happy as one would usually be when an old friend, long separated by time and distance, comes to meet him.
The background of the revelation of this verse Ali 'Imran 3:170 ⧉ as narrated by Abu Dawud on sound authorities from Sayyidna Ibn ` Abbas is as follows. The Holy Prophet said to the noble Companions: 'When your brothers-in-faith fell martyrs at the battle of Uhud, Allah Almighty placed their spirits in the bodies of green birds and set them free. They get their sustenance from the streams and fruit-trees of the Paradise following which they return to special candelabrums held suspending for them underneath the عرش 'throne' ('arsh) of the All-Merciful. When they noticed the luxury of their life there, they said, 'Can anyone tell them (their grieving friends and relatives in the world) about how we live here so that they stop grieving about us and go about striving in Jihad as we did.' Allah Almighty said, 'We shall let them know about this state of yours.' Thereupon, this verse Ali 'Imran 3:170 ⧉ was revealed (Qurtubi).