Freud starts with Schiller’s quote that hunger and love are what motivate the world. He thought hunger represented the preservational instincts (ego), love strives after objects (libido) and preserves the species.  

These two instincts confront one another, the ego and the libido (directed towards an object). But one of the object instincts, sadism, stood out in that its aim was far from being loving. It was attached in ways to the ego instinct, having a close connection with mastery instincts that had no libidinal purpose. But these were ultimately chalked up to neurosis- the outcome of the struggle between the two instincts in which the ego had won, but at a price of severe sufferings. Most analysts consider this basically true today, but it became necessary to alter things as our knowledge advanced from repressed to repressing forces. The decisive step forward was the introduction of the concept of narcissism (self-centeredness arising from failure to distinguish the self from external objects).

But he reasoned that this was not sufficient explanation. He then worked from biological parallels to the conclusion that there is an instinct to preserve living substance and join it to ever larger units, and a counter instinct to dissolve those units. Both an eros instinct and a death instinct.

He found it fruitful to think of the death instinct in humans as diverted towards the external world through aggressiveness and destruction. In this way, the death instinct could be pressed into service by eros, in that the organism is destroying some other thing. Conversely, limiting it’s outward destruction would turn the destruction inward.