>what do people pray for and why do they think they'll get it?
People pray for a lot of things, as you intimate, but not everyone will get what they want. You have to pray properly and not just for anything and assume you will get it, because it won't be given you (Jas 1:7). Personally I pray often for mercy, and to be merciful to others, humility, thanksgiving, for the increase in virtues which I lack, and to be better in prayer.
>But when it comes to less quantifiable things, like health or a good wife or virtue or whatever people still pray for these things as if they can get them. But they can't.
Perhaps because they think prayer is enough, and aren't doing penance for their sins, and aren't persevering in prayer either. Prayer isn't just about celestial gibs. Knowing the Lord knows what you need is not a valid reason getting mad when God doesn't magically give you sick gains overnight.
Then again, we are also given crosses to bear, so loneliness, poor health, a bitter wife etc. will be just these for many of us, so instead of praying for God to take it away from us, we should be thanking Him, and asking Him to help us to bear our crosses.
In fact the idea of physical fitness is an apt metaphor, not least because it is often found in Scripture (St Paul and St Peter refer often to atheletes, races, trophies etc. as allegories of spiritual keenness), but in the same way it's not just one thing. If prayer is akin to weightlifting, then you will not be "fit" by this alone, but though other thing, such as diet, lifestyle changes, change in mental attitude, changes in company, etc.
>Praying is purely to praise God and humble yourself, right?
Not just that. Prayer has four ends; adoration, contrition/penance, thanksgiving and supplication.