Death, the personification of death and the grim reaper, is in love with Deadpool because she cannot have his soul due to him not being able to die. Deadpool reciprocates his love for her and so tries to figure out ways for him to lose his healing ability, die and be with his beloved. Thanos, jealous because he understood Death prefered Deadpool's love over his, cursed Deadpool with immortality so they couldn't be together.
Deadpool and Death meet in the following comics.
Deadpool (volume 3) #15 (fantasy)