Well the ultimate thermodynamic explanation for why the universe is spontaneously dissipating free energy is to be found in the second law and the expansion of the universe, considered to be brought about by the big bang. The fact that these are distinct principles is a subtle point that is somewhat difficult to appreciate. The second law is essentially an informational principle, which says that the most stable macrostates of a system are those which maximize the system’s microscopic degrees of freedom subject to boundary constraints: those electrons hate to stay put. So in less technical terms, the fundamentally random wiggliness of the micro-scale is the means by which the most probable macroscopic states of a system are made actual in time. So in its most abstract sense, the second law can be understood as simply formalizing the (almost) tautological notion that as time passes lots of random motion has the outcome of making the probable into the actual. Without fussing too much over these details, the main point is that this purely statistical/informational form of the second law would be true in any possible universe because it depends on nothing more than probability and a means for a system to explore its possible states.
When we add to the second law the conditions of cosmic expansion brought about by our coming after the big bang, we arrive at the rationale for why the probabilistically spontaneous direction of events we observe is always in the direction of the increasing dissipation of potential energy. It is because cosmic expansion maintains a disequilibrium between potential and kinetic forms of energy (Wicken, 1986). Note that the term “potential” itself already conveys the sense of time-directed dissipation. To understand this simply recognize that the flow of energy requires a potential difference. The sun radiates energy into space because it is hotter than space: the rationale for the radiation is not contained entirely in the structure of the sun itself, but in the relationship of the dynamic possibilities of that structure (nuclear fusion), to the (colder) environment around it. If you put the sun in a container that was already 6000 Kelvin there would no longer be any reason for it to radiate. So if space were a fixed rather than an expanding volume, then existence would have come to thermal equilibrium already. The term heat death would then probably be more appropriately described “gradient death,” because it actually makes no difference how hot or cold existence is, once its the same temperature everywhere then all spontaneous processes will cease.