Our growth in Christ is not in the direction of independence; it is growth in dependence upon God.  A picture of this dependence is given in John 15.

Our Lord Jesus is the True Vine.  We are the branches.  Apart from Him, we can do nothing.  True mourning comes when we realize our need for Him and is in itself a prayer which draws upon the grace of God.  He empowers those who mourn.  The "comforting" is actually the Holy Spirit gracing the mourner with His presence.

Blessed are the "beggars" in spirit.  Theirs is the kingdom of heaven.

Kingdom mourning is the heart's cry for the help of Christ.  Without His help, His disciples are truly helpless, but our Lord has given His promise ... His Word; the true mourner shall be "comforted". 

If we connect the "poor in spirit" with the "mourner", we could say:

   The Holy Spirit will personally come to strengthen and help the one realizing that he is helpless apart from Christ and whose heart cries out for Him.