I'm dragging it a bit off topic, but why does R/T come off as more mother/son?
Going off topic just to answer this question.
Since I've been on RAF, and after having read some interesting topics, I've come to see the Rachel/Tobias relationship in a new light. As a kid, I thought they were a good match, romantically. But now I see them as having a different kind of relationship. Specifically that of a mother and son.
Tobias was bullied A LOT in school, and after he becomes a nothlit he finds it difficult to adjust. Yes, eventually he comes to terms with his conflict of human/hawk, and gains independance, but prior to this revelation he relies a lot on Jake and Rachel to take care of him; especially the latter.
Rachel seems to enjoy the role of protector--it's a motherly instinct, but also stems from her role as older sister. I cannot say why Rachel fell in love with Tobias--maybe it's the whole Angel factor; a dark, brooding, mysterious boyfriend that mom doesn't know about. But she is constantly protecting Tobias in a way that I think is motherly, rather than girlfriendly. Tobias gives off this "needy" kind of vibe that must have grabbed his attention. Maybe that's why she likes Tobias, rather than Marco. With Tobias, she can provide him with the love he never had growing up; she can support and love him. With Marco, Rachel may not be able to do that, simply because he's more emotionally stable--arguably.
Just a thought.
I really think that is the essence of the relationship, now that I think about it. Rachel always was the "momma bear" type of protector, whether it was her family or the world. "Screw you, you mess with my friends I'm going to take care of you myself." Tobias seemed to need that because he never had it, and it was really the only way into Rachel's emotional side (as opposed to "hey babe, you're hot, wanna date?") Rachel felt herself when she was protecting something, and that was how the relationship with Tobias worked. One of the best things the series did, I think, was show the darker side of the relationship (even though I think it was probably the favorite.) Even though the love might seem 'realer' on a level, there is a lot of unbalance there too. She felt she 'knew' what was best for Tobias, and so she always was trying to trap him as a human, constantly worrying about him.
As for Cassie and Jake, they kind of mutually needed each other as well. Cassie grounded Jake from spinning into unchartered moral territory that he wasn't comfortable with (which Marco and Rachel was.) And Jake provided Cassie with a sense that he could keep the world from spinning out of control into the moral black-and-black. In the end, when the morals proved to be what was holding them back from claiming victory in the war, their relationship deteriorated because they lost what was needed to hold them together.
And with the brief Ax romance, he simply needed to know he wasn't the only Andalite in the Universe.
Marco was more or less independent from the get go, probably because he'd already walked through the "I'm on my own" phase with his mom dying and his father going basically mentally MIA on him. He didn't really need anyone to cling to, and was in the end more focused on the war, so he really didn't have the time or need to actually seek out someone on an emotional level.
**of course, this is all opinion.