Toward the end of Season 4 (e.g., Episode 17), it’s completely clear that Monica is the one who pushes Ross into Emily’s arms. I just watched it. She explains to Ross that this is what he’s always dreamed of.

In other words, it’s obvious that Ross doesn’t want any of this. In other words, it’s no coincidence that he later says Rachel’s name instead of Emily’s.

At one point, even Rachel encourages him regarding Emily. But the strongest influence comes from Monica:

  
"\[Scene: Chandler and Joey's erm, Monica and Rachel's, Ross is entering.\]

Ross: Hi.

Monica: Hi.

Ross: So uh, Emily just went to the airport.

Monica: Oh. Why didn’t you take her?

Ross: Eh, her-her uncle already had planned on doing it. And y’know, we-we said our good-byes this morning, so…

Monica: You must feel horrible. Hey! The guys have free porn!

Ross: (Thinks about it.) Nah.

Monica: Hey, cheer up! You’re gonna see her again, right?

Ross: Well I, that’s the thing, I don’t know! I mean, whenever I brought it up with her she said, (In a British accent.) "This is so fantastic! Why do we have to talk about the future? Let’s just enjoy…"

Monica: (interrupting him) No-no-no, don’t-don’t do the accent. You’ve got to see her again.

Ross: And why do you care so much?

Monica: Because! You could get to live out my fantasy!

Ross: You had fantasies about Emily?

Monica: No! Y’know, the fantasy! Meet someone from a strange land, fall in madly love, and spend the rest of your lives together.

Ross: Is that why in junior high you were the only one that hung out with that Ukrainian kid?

Monica: Yeah that, plus his mom used to put sour cream on everything!

Ross: Ahh.

Monica: Do you love her?

Ross: We said it was only going to be two weeks, y’know?

Monica: You love her!

Ross: What-what is love really?

Monica: Ohhh, I knew you loved her! Then you need to go to the airport and tell her. You’re probably just gonna catch her just as she’s about to go to the gate. You’re gonna call out her name and say, "I love you!" And she’s gonna say, "I love you, too!" And you guys are going to have the most amazing kiss, everyone at the gate will applaud.

Ross: I am a good kisser.

Monica: Then you two can, can sneak into the cockpit, and things will start to heat up, and then a stewardess comes in… (Ross looks at her.) I’ve been watching too much porn."

"Ross: What-what is love really?"