It all started with Stan, so technically it was all his fault.

Stan left. It was good for him. Gibbs trained him well and Abby was proud when Stan told her they're moving him onto a ship. No more Gibbs barking at him, calling him Steve.
But.
Abby missed Stan. Maybe he was a little uncomfortable with some topics, but talking to him always made Abby relax.

Stan was replaced by Tony. Things were more cheerful and Abby loved Tony.
But things weren't the same.

Next was Vivian. As Gibbs put it: "back to Edgar Hoover building".
Maybe it was better that way. Vivian never quite fit in. She was far to formal for NCIS.

Most importantly, Vivian didn't like Apocaliptica and it was something Abby considered a crime.
Not that Abby didn't like her. She just preferred agent Blackadder to stay away from the lab.

And Abby liked Kate better. Kate made Viv's desk look nice. And she liked dark lipsticks.

Abby liked Kate. That's why Kate's death hit her so much. Truth be told it was Abby's first death at NCIS.

Ziva, Kate's replacement, came and went, and Abby couldn't really say if she missed Officer David. Ziva appeared while Abby was still grieving and she returned to Mossad soon after.

Abby was a Goth. She wasn't afraid of death. On the contrary, she was looking forward to that experience. Grieving after Kate's death was more about Abby feeling lonely than about Kate's passing.
Abby believed that there was a better place. And there was no reason to feel sad because Kate went there.
There was sadness because Kate was no longer with Abby.

Abby knew that everything has to end. That everybody, sooner or later, has to move on. And she accepted that.

But she really understood it after Tony left.
When he broke his two years record, she started to feel safe. She took his presence for granted. After all, he wouldn't be able to find a better team. There was no boss like Gibbs. And Abby knew Tony adored her. He even found Ducky's anecdotes amusing.
Despite what he was saying, Abby knew better. It wasn't NCIS Tony liked so much. It was the people he worked with.
Madam Director wanted to make him a lead agent. Wanted to send him away. But it had a totally different effect.
Abby never saw Gibbs hugging anyone who wasn't her. Until that day. She had no idea who was more shocked by this hug; Gibbs or Tony.
But at that moment Abby truly realized that everything has to end. Good days especially.
Abby was painfully aware that it was just a matter of time till Tim will be given his own team, till Ducky's retirement or Gibbs' death in the line of duty.
Abby was a Goth, she accepted all kinds of endings.
Didn't mean she liked them.