Basically, martingale strategy is all about recovering our losses which will be possible only when we double the betting. So, if you hit profits then you must move on to test your luck again. If you hit losses then you should work on recovering your losses. So, it makes sense to double the betting after losses and sticking with same bet after profits.
This must be one of the best explanation about what is martingale strategy. I just want to add a little more from what I have observed with this strategy. If you notice, almost all the strategies are working on eliminating losses but making profit is just based on our skill or luck factors.