The Gemara in Bava Basra (60b) quotes a beraisa: וכל המתאבל על ירושלים זוכה ורואה בשמחתה שנאמר שמחו את ירושלים וגו' - “Anyone who mourns for the destruction of Yerusholayim will merit and see its joy, as it is stated: ‘Rejoice with Yerusholayim, and be glad with her, all that love her; rejoice for joy with her, all that mourn for her’ (Yeshayah 66:10).”
Seemingly retributive justice — he who mourns the lack will see the rebuilding; he who has made peace with the destruction of the Beis HaMikdosh – won’t. But there is a deeper understanding of this. The Maharal (Netzach Yisroel chapter 29) explains that the world was created with the capacity to reach completion, shleimus, on every level. That is the purpose of creation – to give birth to the world’s potential of perfection. This will be realized only with the coming of the Moshiach, when the world will be perfected.
One who wishes and yearns for this perfection with acute awareness of the present lack, connects himself spiritually to that future time and existence. As such, he not only has the physical abilities of reaching completion, he also has the connection – in his heart: he is part of the futuristic world. As such he will find himself meriting to see its joy.
Similarly, the Ramchal (Ma’amar Hakivuy) writes extensively on this topic, explaining what hope does in the kabalistic realm, adding that one who hopes for the geulah, even if he is not worthy of it because of his behavior, nevertheless the simple hoping makes him part of the geulah.
A person who hopes and yearns for salvation is part of that salvation, and one who is not interested in it, does not – because he has no part in that world. Therefore, it is clear now why one of the first questions asked by the Beis Din Shel Mahloh is if one has connection to the world of salvation and redemption.
