why is esther in Bell Jar suicidal?

I've read the book and just wondering what really caused her to think about suicide? Was it marco? or was it something else?

Esther was severely depressed. You can see the first signs of her depression when she starts crying for no apparent reason when they try to take her picture with the paper rose. Her illness progresses in her inability to pack up and make simple choices and more obvious signs appear when she returns home and her insomnia begins to take place, she can't bring herself to do anything and so on and so forth. There is no clear trigger for her depression and there doesn't have to be.