Why is the Bourgeoisie involved in a constant battle?

To understand answer to this question we need to understand the concept of class struggle proposed by Karl Marx. As per this concept, since the beginning of human societies with division of labour, societies have been divided in two broad classes the dominant class and the dominated class. There is always a struggle between dominant class and the dominated class for power and for better share of the outputs of the society. Down the ages it has been observed that with major changes in technology, the dominated class may gain ascendancy over the dominant class and itself become a dominant class with a new dominated class emerging under them. In this way Karl Max identified the following pairs of dominant and dominated classes that emerged and disappeared down the ages.

Free men and slaves

Patrician and plebeian

Baron and serf

Nobility and bourgeoisie

Bourgeoisie and proletariat

As described by Karl Max, bourgeoisie is the class of people owning and managing the means of production. This class emerged with the development of technology for production where the tools of production became an important factor of production, as distinct from labour. Proletariat is the class of people who sell their labour to the bourgeoisie and receive wages from them. The product from the labour of proletariat is owned by bourgeoisie and they earn profit by selling it. This profit is far in excess to their own contribution labour in producing it.

The bourgeoisie, when they initially emerged, were among the dominated class under the nobility. But the new technology gave them power, which they used in their struggle with nobility and gain ascendancy over them.

Thus emerged an new class structure of bourgeoisie as the dominant class and proletariat as the dominated class. Currently as per Karl Marx these two classes are in a struggle to gain power. One sign of this struggle is visible in the labour trade union activities. Advent of communistic form of government is a revolutionary form of this struggle.

It is envisaged by Karl Marx that this current class struggle will come to an end with proletariat gaining ascendancy and then establishing a class less society. How accurate these views of Karl Marx are is a matter of debate.