Because unrequited feelings lead to...difficult situations and impropriety.

[It might be a cliche, but it was also a cliche because it was true. People got stupid when they had these sort of feelings. And if there was one thing Ignis couldn't stand, it was otherwise intelligent people acting stupid. So, again, he's choosing his words with all the care of a mountain climber balancing on a small span of vertical rock that's also on fire.]

I've known you almost my whole life, Noctis. I know how...impetuous you can be. And I don't want our...friendship to end because we rushed headlong into something.