This is a promise Jesus made to his disciples. This is a promise to which Jesus' disciples have continued to cling. It is a promise which demonstrates Jesus' care for us and compassion toward us.
It is not a promise which Jesus fails to keep.
We sometimes fail in our application of this promise. Jesus is put to the test - asked for things which would satisfy our curiosity or prove our convictions. This promise is mis-heard as a wishing-wand or as a magic trick.
Jesus wants what we want. Jesus touches the sick and transforms the lives of the broken. John 14:14 encourages us to "ask," but Jesus already knows and Jesus is as eager as we to see the hurting stop and the world made right.
So Jesus makes this promise to us. And this promise resonates in our ears and in our hearts and in our lives. We cling to it and see in it just how precious we are to the Word made flesh, to the creator of the cosmos.