What is the reason Jenny left Forrest without telling him after sleeping with him?

I'm shamelessly going to steal some wording from an article that does a great character analysis of Jenny.

“Instead of realizing that narrative even exists in the story,” Namtara says, “people just bitch about how Jenny is such a slut, but she won’t even love the only person who cares about her. Jenny always loved Forrest, during the whole fucking movie. She loved him so much, she thought she was taking advantage of him and ran away for his sake. She didn’t realize she was wrong until it was almost too late.”

Nailed it. Jenny’s push-pull relationship with Forrest is not about indecision or an easy ride, but the spiralling decisions of a woman suffering abuse from all (but one) of the men in her life. It’s this crucial, crucial fact that “Worst Character Ever” arguments seem to miss entirely – and it’s the most important one of all.

I highly recommend reading the entire character study here in order to understand Jenny's character.

I always thought it was to spare him of her life. Jenny is pretty free-spirited throughout the movie, while Forrest is always more on an even keel. I believe that she felt like her lifestyle would cause him grief or anguish as it had in the past (think "Black Panther party"), and she didn't want him to have to be introduced to that.

I'd definitely agree with sanpaco's statement that Jenny loved Forrest her whole life, and that's why she cared enough to rid his life of her. She only re-introduced herself into his life when she knew she was dying and realized Forrest was the best person to raise their son.

In my opinion, Jenny’s departure from Forrest’s house, AND her “No” answer to his proposal for marriage are inextricably linked.

It’s obvious that Jenny and Forrest loved each other as best friends from the time they first met. There was absolutely nothing that one of them could do that would break that bond with the other.

But did Jenny love Forrest like a woman would love a husband? Yes is my vote, and here is why. From Day 1, Jenny saw into Forrest’s heart and soul – beyond leg braces and beyond low IQ – and saw a good, good man. So why say “No” to his proposal? It’s because she did not see herself as worthy of being Forrest’s wife. She is the only one in the world who knows just how much her life’s conduct has driven her away (and below) from being a worthy wife. Please look at the dialog between them at his proposal, or view this YouTube clip.

Forrest: Will you marry me?

(Jenny slowly turns around to face Forrest without expression.)

Forrest: I’d make a good husband, Jenny.

Jenny: (Matter-of-factly) You would, Forrest. [Emphasis mine. She means it – this is no kidding.]

Forrest: But you won’t marry me.

(Jenny shakes her head no with a disappointed sigh and expression)

Jenny: (Shaking her head no) You don’t want to marry me. [Emphasis mine – this is crucial.]

Forrest: Why don’t you love me, Jenny? [Forrest has it wrong.]

(Jenny goes to respond, but doesn’t find the words.) [Because she is conflicted.]

Forrest: I’m not a smart man. But I know what love is. (Walks away.)

Later on in the clip, Jenny comes to Forrest’s room:

Forrest: (Surprised) Jenny?!

Jenny: Forrest I do love you. [Emphasis mine]

(No more dialog as this is where Little Forrest is conceived.)

Aside from Forrest’s understandable surprise, these are exactly the actions that a husband and wife would do. Jenny knows all too well how to practice birth control. And she knows all too well that she is not doing so. She owned up to the only part of being a wife that she would allow herself to attain.

So why did she leave?

As loving friends, she probably would have stayed quite a while. As a prospective wife, she could not bear to be in his presence. She did not see herself as being in his league. In her mind, she had to leave. That was her loving gesture to him so that he could find someone who was worthy of him. She knew he would not understand, so she left in a manner that would cause the least pain to him.

Unfortunately, it’s only until she is dying that she realizes that Forrest sees her heart and soul as much as she sees his. It’s only after he offers to take care of her that she realizes that that’s all he sees: a great person that has always been worthy to be his wife. And that is why she proposes to him. Of course, he accepts.

3 reasons why Jenny leaves Forest the night after making love to him and telling him she loves him:

1. She realized he was the 1st man who never judged her, never tried to control her, never abused her. She did love him in her own way.
2. In her mind she had a very ugly past, was mentally unstable and she was not going to burden him with her past, a past she was not ready to confront.
3. She would always love him but she couldn't be the woman that he deserved and she wasn't going to interfere, give him the opportunity to find the kind of woman he deserves.