He's clearly confused about how you're just completely disregarding what he must see as a threat to his guardian. Shirou can probably tell that something is magically off about Taiga thanks to his strange way of determining whether magic has been used. It's his sense of smell that is affected by the use of magecraft if you remember correctly. Of course, your own way to detect the use of magecraft is far more advanced in that you can usually simply tell when it has been used if you're looking at it. Of course, this does leave you somewhat more vulnerable to being hit by things that take you by surprise.

Part 10