We all know Spike and Twilight has the bond of brother and sister, but now they(mostly Spike) just drift away from each other.
 
Spike started resenting and rebelled against Twilight, cause she won’t let him grow up or go anywhere without her permission, refuse to let him tag along on any missions cause he has no special abilities or he’s not a chosen barrier of the Elements, it left Spike feeling angry that Twilight won’t let him learn or do anything and he feels trapped living in her shadow. Which led him to want to get away from Twilight and her world. But Twilight’s not the only person he’s angry with, the other girls are part of the problem as well.
 
In Twilight’s case she’s afraid to let Spike grow up cause she loves and cherish him with all her heart and felt that he would leave her life forever, What made her became really so overprotective over him is that when Spike tag along on a mission he got hurt badly and Twilight blamed herself for it, that’s why she forbid him for coming anywhere with her she even said some harsh words to him to make him get it, Although she felt guilty about what she said, but she felt she only did it to protect him, When she try’d to apologize Spike ignored her and wouldn’t talk to her. Twilight started seeing that Spike was changing as he grow up and it left her scared as she miss the little boy who was her number one assistant, friend, and little brother.