Why do Marx and Engels believe that the bourgeoisie are doomed and that they will become their own destruction?

The bourgeoisie are capitalists who are interested only in the personal accumulation of wealth at the expense of the ordinary worker. The bourgeoisie will keep exploiting the ordinary worker more and more to accumulate more and more wealth. This will create a larger class of workers and will create a larger and larger disparity in wealth between the bourgeoisie and the proletariat. Eventually, the class of the proletariat will become so large and so dissatisfied with their lot in life that they will revolt against their masters and take away the means of production of the society. Marx felt this was an inevitable force of human nature and that this was the inherent contradiction in capitalism that would eventually cause its own destruction.