Nygren argues that eros is an egocentric and acquisitive kind of love, needs-based and desire-based. When we love out of eros - whether we love a god or another human being -, we love out of self-interest and in order to acquire and possess the object of our love. This form of love received its classic expression in the philosophy of Plato, particularly in his dialogue The Symposium.
This is the common definition of Eros, and even Aristotles said that god himself is Eros, and that everyone point their love to the heavens because they desire.
Agape, by contrast, is a self-giving and self-sacrificial kind of love. It is based on God's unconditional love for all creatures. When we love out of agape we reject all self-gain and interest, and surrender ourselves to the other and love them purely for themselves.
God let Jesus die to a bunch of sinners due to his unconditional love (agape). God's supposed to be seen as a figure which we cannot understand because we could never go into a bandit's lair and watch all of them kill a good person and still love everyone equally, but god could. People think this is what unconditional love is in its highest form, and it's true, it totally is. By this definition dead people are closer to true love than us who merely have biases (eros) to people or things we love for personal reasons.
The problem is, this definition of agape is not at all for humans, or any sentient being to have. It entails a philosophy where you don't invest yourself personally
into anything, but rather everything on an impersonal level. This is agape; this is the sort of unconditional love that allows you to forgive the person that's raped the people you care about, or the people who constantly do evil in this world.
When I show you the clear evilness in the perceived meaning of agape and really the whole pillar of Christianity (love and forgiveness), it's easy to think that agape is a bad thing used by depressed people detached from their humanity, or some jedi or monk, but I'm just pointing out the flaw of this type of love. Does that mean Eros is the right type of love for us lowly beings?
Eros which is desire, corrupts us. We change who we are to get people we like to want us all while denying our true selves. Eros is why people do stupid things like gamble, rape, immoral actions for the sake of an outcome etc. It's easy to see why people turn to agape when Eros exists. Agape is too far away for them to grasp, and eros is too prevalent in the evil corners of history starting from when Eve took a bite of the apple.
Eros is indeed a bad thing that needs to be controlled, but without it there is no passion or driving factor. It is said that eros is what motivates us. Agape should only be given to the things we come to love naturally for moral reasons. If we have a bias to certain people that isn't corrupting us then surely they deserve this agape.