Potatoes, pasta and boiled jasmin rice are all starch and do not taste much by themselves. However I would not serve tomato sauce nor olives to the rice. On the other hand I would not serve potatoes or pasta with soya sauce.
Why is that?
Also oven roasted potatoes may be good with ketchup and even mustard. So oven roasting "transforms" the starch in the poatoes somehow.
Likewise frying the rice (pilaf) make it "fattier"/"heavier" than boiled rice and the range of what you can add to the rice increases. In this case fat is added but I think it is the "roasting" that is the main thing.
So to me it seems that there is some property of the starch. For lack of better words I will call this roastedness.
Roastedness(potatoes) > Roastedness(pasta) > Roastedness(rice).
Oven roasting or frying increases roastedness for all.
What is this property I call roastedness?
May pH have something to do with this? Tomatoes are acid. Rice is slightly acid. Potatoes are slightly acid except for the skin which is alkaline. On total potatoes are therefore slightly alkaline? Soya is alkaline. Oven roasting or frying potatoes produces acrylamide which is alkaline? Combining alkaline (bitter) and acid (sour) taste good.