It is easier to maintain the habit of exercise if you do it in small chunks 6 days a week, rather than 2-3 times a week. Even then, common human laziness slows us from time to time. During these moments of low motivation, having a written list of personal reasons to exercise (which you can refer to as needed) is helpful for self-encouragement.
Here is my current list of reasons:
To be able to wear midriff shirts because I have a sexy, flat stomach.
To wear size 4 jeans and tell EVERYONE.
To have the option of wearing cute designer dresses, even if I don't.
So no one ever again mistakes me for being pregnant.
So I can take up extreme sports without giving myself a heart attack (boxing, martial arts, and freerunning/parkour).
Be less tired day-to-day and have energy to do daily tasks.
So I can study and think better.
To ward off Alzheimers.
Sleep, Ahhh, glorious sleep. A good night's rest is worth gold.
Being one with my body will put me more in touch with my spirit and spiritual side.
Make friends in my exercise classes (I plan on taking yoga to start).
Because it is embarassing when my retired father outwalks me at the mall.
So I can go back to dance class, which I miss sometimes. Also so I can go dancing.
To open any and every jar at will!
To give myself a better chance of recovery in case I ever experience a traumatic injury.
To encourage my boyfriend and family to also get into better shape.
If I exercise more, I can eat more good food.