@Cycloptichorn,
You've already clarified that by "dead human" you meant not Christ, but
Mary. Thank you.
As for our squaring our predilection for praying to dead humans with
Jesus' actual teachings, I think it comes down to three things. First,
Jesus taught the efficacy of prayer. Second he spoke of a a life after
death. Third, he urged us to help each other. If prayer is efficacious,
and if we wish to help each other, then it stands to reason we would
pray for each other. If dead humans survive in a life after death, then
they could pray for others as well. So we ask them to do so.
Now of course all these "if" clause make some pretty big assumptions.
I'm not trying to defend any of them. I'm simply trying to explain why
we do some of the things we do.
Having said that, intercessory prayer is prone to abuse. That is why I
made the distinction between praying to God and praying to dead
humans to ask them to pray to God. (Bear in mind that we are only
speaking of prayers of supplication here.) The danger is of praying to
a saint
as if he or she were God. Sadly, this happens among some
Catholics and was one of the things Luther condemned and sought
reformation of.