There are loads of reasons you can get a flat battery. You can get any bike going by bump-starting it, but some are harder than others. The BMW G650 Xmoto of my girlfriend, 650cc single cylinder engine motorcycle is one of the harder once.
I have some pointers: A hill is your friend if you're bump-starting, don't say 'I know', it's incredible how many people know about the hill but still not use it. Even a slight gradient will make a difference. Point the bike down it to help you pick up speed. If there's no gradient, a mate can assist by pushing the bike. Grab a passer-by if necessary, they are for females easier to find then for men, but still.
If the bike's cold, set the choke to the normal start position. Test to see if the bike can be pushed while in gear, second gear is mostly the best, with the clutch in.If not, you'll have to push it in neutral then flick it into gear as you're about to bump it.
Get the bike rolling as fast as possible. If you've got a hill or a helper, you can sit on the bike, otherwise you'll probably have to run along side it as you push to get enough speed up.
Once it's moving fast, jump on if you're not there already. As your bottom lands on the seat you need to dump the clutch with the bike in second gear. If you're sitting on the bike, stand up on the pegs and sit down hard as you drop the clutch. This helps stop the rear wheel locking up.
Single-cylinder four-stroke bikes can be a sod to bump-start. Prepare the bike by pushing it backwards with it in gear until the engine locks completely solid. You've then at the right point in the cycle to give you the best chance of getting the engine started.
The best technique, especially if you don't have a hill or helper, is to have the bike in second gear, clutch in and push by running alongside it. Then, rather than jumping right on to the bike, just drop your bum on side-saddle as you dump the clutch. It needs good balance but it means the bike's traveling as fast as possible when you try and start it. This technique is not advisable for newbies, or people that rented a unfamiliar bike.
Take care, if the bike fires up it may suddenly accelerate off. So pull the clutch in and give it plenty of revs until you're sure it's running OK. Turn off or disconnect the headlights and hopefully the battery will get enough charge to start the bike after riding for 10 to 30 minutes.