There are lots of people out there that play the occasional football or tennis match or spend a week per year skiing. Most martial arts require a different approach and cannot be practiced occasionally if you want to enjoy the benefits that they can bring and avoid injuries. While I adopted a regular, quasi religious training regime since I was a teen ager I see many of my students or other fellow martial artists having a very irregular training regime: I believe this can be the strongest cause of injuries and loss of motivation.
When you are at the beginning of your training you have a steady increase of performance in terms of speed, power, flexibility and, progressively, technique. Your mind, as well as your muscles, get trained and they learn the subtle intricacies of how and when firing the right muscles in the appropriate time and order. You can consider that some of the muscles used in certain techniques are not used much in our normal daily activities. For the same reason these muscles have a stronger tendency to loose their performance when not used.
While in regular training you enjoy progresses in your training and this enjoyment is released in the form of endorphins that make you feel good. If, for any reason, you stop training for a few days or weeks your muscles tend to loose some of their fitness. When you try a technique that was nice and easy last time you did it you find yourself suddenly struggling with it or, if that happens in a self defence situation, risk your life in the process.
A regular practice for amateurs should be considered when training 2-3 sessions per week, possibly practicing all year round: each training session should be between 1 and 2 hours long.