When people train on their own, their results will last as long as their own, personal motivation. For some people this is all that they need, as their own motivators are strong enough to push them all the way to the end. For others, however, it can result in progress stopping, and often efforts also stopping.
One of the best ways to prevent this from happening is to train with someone else, or a group of other people, who are looking to achieve the same goal as you are. When your own motivation starts to slow down, you have the rest of the group to pull you back up and continue to push you towards your goal. This is especially true in Group Fitness and Bootcamp fitness – the group will almost become a second family, with each person looking out for each other to make sure they are staying consistent to achieve their goals.
Training with someone who has a vested interest in you achieving your goals, such as a Personal Trainer, will also greatly increase the success you have in achieving your goal. This is why people that see a Personal Trainer in some capacity will get better, faster results!