One: They depend on the opinion and feel motivated only when others say something good about them; as in like, you have lost some weight, or you look fitter than before.
Two: They see how much farther they have to go still and not how far they have traveled and the changes they have made. Small changes matter a lot too, remember little drops of water make a mighty ocean. The small changes keep on adding on. In the end, it gets you to where you want to be.
So, always keep track of the changes you have made, no matter how small it may be on the weight scale or how you perform in the gym. Remember each change matters, no matter how small. Something is better than nothing.
One way to keep track of changes is to write down your lifts in the gym on compound movements like bench press, dead-lift, etc. Gradually try to increase weights using progressive overload, and every time you reach a personal record, PR, note it down.
Another way to keep track is to take photos of your physique in the gym. There is no greater proof than visual evidence, but make sure that you give enough time between the photo sessions, a minimum of 4 to 5 weeks at least, to see your progress.
The third way to keep track is by using measurements. Take measurements of your muscle groups, and note them down in your diary, kept specifically for this purpose. When you have made noticeable improvements and increased numbers on the weight scale, measure again, which will be a telling tale of your progress and keep you motivated.