Nas himself has said he initially wasn’t going to respond to Takeover because he simply didn’t care. His mother was sick, he was going through shyt with Carmen, he and Steve Stoute weren’t seeing eye to eye, etc.
Jungle is the one who convinced Nas to respond, that and the fact that the streets were telling him Takeover was a career ender. The streets just wouldn’t let it go so he knew he had to go to war.
Nas never thought much of Jay as an MC, he had to have his eyes opened to how much Jay had ascended in the eyes of the Hip Hop culture. Once he grasped just how serious the situation was he went all in.