Lucifer offers John a favor for helping stop his son from ruling Earth.
Lucifer wants John's soul for all of the demons he's sent back to hell, but Earth is the overall goal of his ambitions.
Lucifer explains his motivations for the exchange in question below.
Lucifer: [To Gabriel] This world is mine, in time. You, best of all of us, Gabriel, should understand ambition.
Lucifer's son was attempting to work with Gabriel to come to Earth through Angela Dodson via the Spear of Destiny. Had Gabriel pierced Angela's stomach with the spear, Lucifer's son would have reigned over Earth instead of Lucifer.
Gabriel: [Gabriel sits on top of John Constantine, explaining her rebellion] You're handed this precious gift, right? Each one of you granted redemption from the Creator – murderers, rapists, molesters – all of you just have to repent, and God takes you into His bosom. In all the worlds and all the universe, no other creature can make such a boast, save man. It's not fair.
[Gabriel leans closer to Constantine's face]
Gabriel: If sweet, sweet God loves you so, then I will make you worthy of His love. But it's only in the face of horror that you truly find your nobler selves – and you can be so noble. So … I will bring you pain. I will bring you horror.
[Gabriel lifts up Constantine from his collar]
Gabriel: So that you may rise above it. So that those of you who will survive this reign of hell on earth will be worthy of God's love.
Constantine alerted Lucifer of the events transpiring in the adjacent room; at first Lucifer is hesitant to trust Constantine.
John Constantine: [Regarding Lucifer's son] He's in the other room.
Lucifer: Boys will be boys.
John Constantine: With Gabriel.
Lucifer: [disgusted] Huh? No accounting for taste, really.
John Constantine: They have the Spear of Destiny.
Lucifer: [mocking, mimicking] "They have the Spear of Destiny." [becomes serious, leans in] Or is it another one of your cons?
Lucifer then stops Gabriel, sends his son back to Hell, and offers a favor in exchange for stopping his son from ruling Earth instead of him.