At some point I must have felt lonely, or happy, or something, because I wanted to talk to someone. (1.36)
When Rachel gets lonely, she gets desperate, and she calls her ex-husband. You know you're in a bad place when the only person you want to call is the person who doesn't want to ever hear from you again.
I waited for Scott to come and calm me down, but he wasn't at home. (4.3)
Megan's abandonment issues damage her relationship. Scott isn't cheating on Megan or anything like that; he's simply leaving for business purposes. Yet she still has an affair so she isn't lonely while he's gone.
"Oh. And she's all alone. That's a bit sad. […] They always go for you, the lonely ones, don't they? They make a beeline straight for you." (4.28, 4.30)
Scott is making a judgment about Megan's friend Tara, but he might as well be talking about Rachel, too. Rachel is also lonely, and she has latched onto Megan… or at least her idea of Megan.
I just want to remain safe and warm in my haven with Scott, undisturbed. (6.1)
Megan is unable to be safe or warm or have a haven by herself. Scott is a critical part of the equation.
Megan has no family in the area. Both her parents are deceased. (7.7)
Even though we only briefly see them, Rachel's mother and Scott's mother briefly appear to offer support to their children. Megan doesn't have that luxury, though; she's alone.
I was trespassing. That's what it felt like this morning, because it's their territory now, it's Tom and Anna's and Scott and Megan's. I'm the outsider. I don't belong there, and yet everything is so familiar to me. (7.21)
Seeing these people so often reinforces how isolated Rachel has become. She used to be a part of these people's lives; now she's a part of no one's life.
I felt isolated in my misery. I became lonely, so I drank bit and then a bit more, and then I became lonely, because no one likes being around a drunk. (11.19)
Here Rachel realizes how damaging her drinking is and explains why it's so difficult for her to stop. It's one nasty cycle.
"She's a rubbernecker. […] Lonely, a bit desperate. She just wants to be involved in something." (18.7)
The way this is stated—from Detective Riley to Anna, no less—is harsh but ultimately true about Rachel. She is lonely and desperate to be involved with something.
"I just waited for him, for someone to come. He didn't come back. He never came back." (22.7)
We finally see the root of Megan's abandonment issues here. One of the first men she cared about—who she also had a baby with—left her and she never saw him again. Ouch.
I'm a hotel in a little town on the Norfolk coast. (38.28)
Rachel gets some time by herself, and it's actually good for her: She stops drinking.