So many things are motivating on a tour, here's a handful that stick out for me:
People are nicer to touring cyclists than they are to other cyclists, not always, but enough to make you smile. People smile and wave at you from their cars as they pass or offer you food and drink (though not while driving

). Often people will let you stay in their homes or at least camp on their land, I think the romanticism of a touring cyclist's life just endears you to them.
You get to see more countries than the average person because your only expenses are food related and maybe camping fees or whatever. It's much cheaper than conventional travel.
You're not working, how can that
not make you happy?
I'm sure riding across the prairies would be dreadfully boring, it's dreadfully boring at 100km/h with air conditioning and a stereo. If I were ever to ride across Canada I would skip the prairies. I'd fly from Calgary to Toronto and skip the unpleasantness.
Food! Even if you do decide to ride through the large, perfunctory expanse that is the plains you'll be motivated by that next greasy cheeseburger and coffee at the next little diner. Or healthier food if you're not in North America. You could also bring an iPod or whatever but the prairies are just toil.
Definitely get out of your car and go touring, North America is good for some shorter tours IMO but if you want to go really long I recommend Western Europe. The food is better (except in the UK, avoid the UK) the architecture is amazing, there's so much more history, there's usually a town every 20 kilometers or so and the people you meet are way more friendly and interesting.
Oh another thing: you know when you're traveling somewhere by car or train or whatever and you see someone interesting? It's a lot harder to meet them when you're rocketing past at 50-120 kms/h On a bike you can just roll up and say "hi". You meet a lot of interesting people this way