The Reveal in chapter 173 that Furuichi knew all along that the tissues granting him the power of demons are slowly killing him. He's just so weary of being the Butt Monkey that he's willing to risk his own life if it means having a taste of what it's like to be one of the main characters.
This could be seen as a brutal case of Reality Ensues, since what Furuichi experiences is technically bullying and many victims develop low self-esteem and become suicidal.
Chapter 207: After chapters of Furuichi showing how much of a badass he was, he got his heart ripped out and died. Temporarily.
Beelzebub is a pretty violent manga, but the gravity of the situation hits home when Tojo sees Furuichi lying in a puddle of blood with a gaping hole in his chest and has a complete Freak Out.
In chapter 220, Lucifer proves that she truly values her time with Takamiya, after she takes a knife blow that was meant to kill him. If that wasn't enough, she then desperately pleads not to be taken away from him, all while he cradles her in his arms.