I'll bite:
Because that's the way God wants it. He desires that we pray, and wants us to be in communion with Him in this way.
And does he want you asking for petty shit, because on the grand scale of things curing your Aunt Rose of her Rickets is a bit petty and that is the kind of shit I see people praying about.
For those of us that are of the Christian pursuasion, we see that Jesus set the example - he often healed people - and never indicated that those things were "a bit petty."
I think that is much more of a valid prayer than: "Please help me get that raise at work so I can buy a new flat screen TV"
Pray for thanks.
The sad part about prayer is most pray only when something is needed.
Understandable, and I'd never truly fault anyone for hoping that someone they love gets better but my point is that if God is infallible why waste time bringing his attention to something he obviously is already aware of and may be part of a much bigger picture that our tiny human minds couldnt possibly comprehend. Plus Jesus basically was basically God and had the power to heal people, we don't have that power and again God knows what he is doing, who am I or you to try to convince him otherwise?