2.5.3: Why do we think that Thom will marry Moiraine? --Updated
Moiraine says she knows the face of the man she will marry better than El/Eg/Ny know their future husbands [TSR: 6, Doorways, 90-1]. This could mean that she will never wed, but it could mean that she really does know who she will end up marrying. Support for the latter case is that, according to Elayne, Moiraine had some passion in her voice when she mentioned a husband, despite her attempts to then brush it off. After going through the Tear twisted doorway, she immediately tells Thom that he will live through the next set of adventures. This is not the only example of Moiraine being so sure of Thom's fate; while the party is traveling through the Ways in TEOTW, Moiraine implies that Min saw something about Thom which makes her think that Thom was not killed by the Fade [TEOTW: 45, What Follows in Shadow, 568]. She certainly seems to have some sort of knowledge of Thom's future. Thom keeps referring to Moiraine as a good-looking woman with more and more sincerity as time progresses and he discovers he no longer loves Morgase.
Possible scenario: How would she know who it was? Being Aes Sedai she'd likely have a lot of chances, but here's a guess. When Min describes how her viewings work her standard example is this: I see two people who have never met and know they will marry-- and of course she had both Thom and Moiraine in front of her in Baerlon. So, this would also be the reason why 1) Moiraine was so sure Thom hadn't been killed by a Fade, despite Rand's and Mat's protestations, and 2) why she tells Thom "I will see you again. You will survive Tarabon." [TSR: 17, Deceptions, 195] At this point, she is absolutely certain that she will see Thom again. This is before she goes to Rhuidean and goes through the rings that show possible futures. When she did that, she saw nothing beyond the point where she tackled Lanfear through the Twisted Doorway. She thus decided that Min (or whatever oracle led her to think she would see Thom again) had been wrong. [Sean Hillyard] Note that, right before tackling Lanfear, Moiraine suppresses a "small bubble of hope," which she feels, even though she is sure she's about to die [TFOH: 52, Choices, 632].
Egwene has a Dream of Thom pulling Moiraine's blue head jewel out of a fire (i.e. Thom rescuing Moiraine from durance vile in Finnland). Nothing sparks a romance (in stories, at any rate) like a rescue. It seems that Moiraine already has tender feelings for Thom from her addressing him as "My dearest Thom" in the letter she sent to him. [KoD 10: A Village in Shiota]
It remains to be seen if Thom returns her feelings, however his eagerness to attempt the rescue is a positive sign in that direction.