I would like to make my kitchen counter tile glossy/shiny. I would like to polyurethane it but have been advised not to do so. Why can this not be done. Also, what other alternatives do I have? Thank you for any help you can give me.

Because polyurethane like any paint will not have the durability needed for a countertop. And it won't bond sufficiently to certain existing countertop materials.

The only way you bond something, except for contact cement, is for the material to penetrate the surface. Tile is too dense. You can try floor wax but it will be temporary.

Hi, patrina!

What are your counter-tops made of, please? It might be possible for them to be sealed with a substance to achieve the effect that you would like, but it would depend on the material out of what they're made first in order to make any kind of determination as to whether they can be coated with anything or not.

Thanks!

Using different products depending on porosity of the tile and grout.

It’s really a question of the porosity of your tile and grout.

At the very least you’ll need to seal cement gout with a penetrating sealer. Latex and epoxy additive grouts are supposed to be more stain resistant but I’d still seal lighter-colored grout lines. Nobody likes dirty grout.

If your tile is a glazed ceramic or nonporous porcelain then you don’t need to seal but may want to just to make cleaning easier, mostly this is done using the same sealer as the grout. Of course sealing will need to be redone periodically. Because of this some people swear off sealing smooth glazed ceramics and only seal grout lines. In my view this is too fussy, I’d rather just swab the sealer on everything. You decide.

Unglazed tile such as Mexican quarry tile must be sealed with a surfacing sealer but I’m hoping you’re not using this type of tile on your counters.

Stone tiles require sealing using the same products you’d use on solid stone counters. Hard stone tiles like granite are obviously less of a problem then porous stone such as marble or limestone. You need to use a breathable penetrating sealer and reseal periodically.
You might also want to click on this link; Sealers - Sealers for Tile, Grout, and Stone - CTaSC
Good luck, Tom