Just question about Juniper..

I thought Juniper's child was fathered by the guy Mud shot.

But to answer your question about Juniper's actions, she was looking for love in all the wrong places. This is a quite common mistake, hopefully she'll learn not to.

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She did love Mud (i.e. she's crying after she gets the letter), but she also notes that Mudd is a liar and a bit of a sleaze. So there was a mixture of emotions there

I think the movie implies that Juniper and Mud had been together in the past, off and on. But things just couldn't work out or didn't work out for them, no matter how hard they tried. Just a waste of time for them to try again.

Remember when Mud recalls the story of how he shot that guy. The kid asks, "Where were you?" Mud says something like, "We weren't on the same side at the time." And Mud's father / guardian. He seemed aware of a long history between Mudd and Juniper. Again - those sort of details hint that Mud and Juniper had an off-and-on kind of thing. They had feelings for each other, but they also didn't.