Well, there's a lot of reasons
I mean, roses only last like a couple weeks
and that's if you leave them in water
and they really only exist to be pretty
so that's like saying
"my love for you is transitory and based solely on your appearance"
But a potato!
Potatoes last for fucking ever, man
In fact, not only will they not rot, they actually grow shit even if you just leave them in the sack
That part alone makes it a good symbol
But there's more!
There are so many ways to enjoy a potato! you can even make a battery with it!
And that's like saying "i have many ways in which I show my love for you"
And potatoes may be ugly, but they're still awesome
So that's like saying "it doesn't matter at all what you look like, I'll still love you"