I understand why some think so. Riley showed Willow more understanding than her other friends, including Buffy herself. And even when Willow was drowning in her grief, she did something unusual and helped someone who was practically a stranger to her which isn't her normal behavior (though she's helpful to those she's loyal to)...more than once.
It was also nice that the pair seemed able to talk to each other like sensible, intelligent human beings without the drama, but then love is a curse in the Buffyverse so maybe the fact they could is proof that they couldn't be in love (or it would go bad fast if they did).
And from a metastory perspective, the writers wouldn't have to
assassinate his character to make room for (quasi) Spuffy. Though it would feel different without Tara or Oz (but at least Riley would strike me as more believable than Tara for coming down hard on Willow for even when magic made sense).
Also, the writers made Riley needy. Well Willow would be as needy as he wanted (and though a witch, at least she's not as strong or tough physically) so it does seem it would work out better, at least going by the story we got. And Riley is pretty smart, not as much as Willow, but enough to keep up (and she'd probably like being intellectually stronger as Riley does in being more capable physically and in combat), and given Professor Walsh, Riley shouldn't have a problem with someone like Willow who would demand a lot of his time and yet also be assertive, potentially domineering at times. (Though Riley could leave for the same reason Tara did, especially if Riley was brain sucked by Glory on top of the mind control chip he endured in s4.)
We should be spared the "cutesy" way she had of talking with Oz and Tara. The more I think about it, the more I'm curious about what could've been.