Exploitative: Labour is the source of all value; therefore, the appropriation of value is the exploitation of labour. In the capitalist mode of production, profit is generated by the application of labour power, bought at a certain cost, to capital, producing commodities which are then sold on for more than the original labour cost (taking into account the overheads of capital). This is described as M-C-M', in which "M" is the original investment, "C" is the commodity produced, and "M'" is the exchange value of that commodity. M is comprised of the costs of capital and the costs of labour, while M' is comprised of both of these, and of additional value, "surplus value". As capital cannot create value- it can only facilitate the creation of value- that surplus value must have been created by labour. Yet the labour power, the commodified form of labour, was bought for less than the value it generated; a part of the value of the labour is never returned to the labourer herself, but is instead snatched up by the capitalist through uneven commercial transactions; it is appropriated, and therefore exploited.
That's a simple summary, of course; a video offering a more detailed explanation can be found
here.
Oppressive: Humanity is a labouring creature-
homo laborans, as you say- who makes and remakes his material surroundings, who creates his experience of the world through his labour. Autonomy demands freedom of labour. Thus, for the majority of the human race to be placed in a position where they do not have freedom of labour, but are obliged to sell their labour to a property-owning elite, is an oppressive state of affairs. The attainment or human liberty therefore demands the reclamation of labour as a free endeavour, unhampered by capitalist parasites, through the re-appropriation of the means of production by the labouring masses.
Beyond this, socialists make an additional claim, which sets them apart from individualist anarchists (who is not the same thing as "anarcho-capitalists"), distributists, and so forth, is the claim that production is a social process, and that no one individual's labour can be meaningfully removed from the social context which allows that labour. The capitalists, in taking control of the process of production, usurp this social process, and so not only engage in individual oppression, but in social oppression, that is, of the subjugation of society as a whole to capitalist interests. Therefore, the reclamation of free labour must be a reclamation of
social labour, which means that the means of production must not simply be re-appropriated, but socialised, which is to say, must be placed under the control of the community, dissolving capitalist social relations and thus preventing the restoration of the capitalist class. (The failure of the Bolsheviks to dissolve these relations in Russia, and the subsequent emergence of a state-capitalist regime, demonstrates their necessity.)
Spiritually Destructive: This follows on, to a large extent, from the above. Man the labourer, again, is a creator, and defines himself through the creation and recreation of his material environment. Free people create as they wish, and so are able to strive for self-fulfilment on their own terms, while unfree people do not create as they wish, and are able to strive for self-fulfilment only- in the majority of cases- in the margins of their productive experience, if at all. They make seek satisfaction in their work, or through hobbies or volunteering, but ultimately they are bound to the profit-motives of capital, which pays no regard for individual well-being, and instead places human self-fulfilment entirely secondary to the process of accumulation.
Furthermore, if we accept the socialist proposition that labour is a social process, then this self-fulfilment and self-realisation is also a social process. Capitalism, as a system which atomises individuals and breaks them apart, stifles this process, preventing not only the individual pursuit of self-fulfilment through the monopolisation of individual labour, but by preventing the emergence of social conditions which allow a true self-fulfilment to be achieved. As social liberation is the pre-requisite of individual liberation, social conditions allowing for self-fulfilment is the pre-requisite of individual conditions allowing for self-fulfilment.
