Why were socialists against private property?

Hint: Socialism is a political ideology based on the collective ownership of the modes of production. Though it seems very close to the ideals of communism, it is different in the way society is perceived. While communism calls for a sudden revolution by the working class and the occupation of the industries and the capital, socialism vouches for a gradual change in society and collective as well as state ownership.

Complete answer: Private property is the ownership of a land, capital, mode of production, labour etc. by a single owner. This gives too much financial and productive power and alienates the workers from their own produce. Thus, it causes much grievances in the society.
These grievances are a product of widening economic gaps between the capitalists and the working class. To bridge this, the socialist propose the ownership of property, mode of production, capital etc. collectively by the society. This means, there will be less accumulation of capital and power into the hands of less people and consequently, there will be more equality of resources.
Socialists also promote the acquisition of those properties by the state instead of collective ownership by the people which are primary and eminent for the society as a whole.

Note: The adoption of socialist ideals in the Indian Constitution was not in the originally drafted constitution. This was done in the 42nd amendment of the constitution in 1976. The government then was moving towards the nationalisation of the Maruthi production to commemorate the ideals of socialism under the leadership of former PM of India Indira Gandhi.