Mat 5:4 Blessed [are] they that mourn: for they shall be comforted.
Alternative: Those that mourn are fortunate for they will be summoned.
The word that we translate as "comfort" is parakaleo. This verb primarily means will be "called" (kaleo) "towards" the caller (para), that is, "will be summoned. Death is being called to God, so when we feel loss at a death, we should recognize that we will someday be summoned God to God. More general, parakaleo means that we are being told something. The message can be comforting to us, but it can also be teaching us , begging us, or encouraging us. We have see this word other places such as Mat 26:53, Mat 18:32, and Mat 18:29, where it always means "to pray," "to call," and "to beseech."
Again, Christ meant all these ideas at once. When we mourn, it is because we have lost someone that we love. In that loss, we should see our own future. It is a foreshadowing of our own future summoning when we will be called to join God. This call is at once comforting because death is not final, a lesson about our future, a entreaty to change, and encouragement to persevere.
This is part of the answer to the question," Why does God allow suffering?" that we discuss here under the problem of evil.
"Blessed" is from makarios (makarios) which means "blessed," "happy," and "fortunate."
"Mourn" is from pentheô (pentheo), which means "to bewail," "to mourn," "to go into mounring," and "to lament."
"For" is from hoti (hoti), which means "for what," and "wherefore." A form of hostis (hostis), which means "that," "any one who," "anything which," "whosoever," "whichsoever" and "anybody whatsoever."
"Comforted" is from parakaleô (parakaleo) which means "call in," "summon," "address", "demand," and "beseech." It means literally "call closer."