What to say to your boss if they want you to use a global variable

Question

I am currently 4 months into an internship, and when reviewing my code, my boss didn't like that I had kept a specific object local to a number of methods across a few separate classes within one assembly. He didn't like that I was created a new object each time, and instead told me to create a single object that can be accessed from anywhere. I have therefore had to create it as a static object within a static class, and simply reference it from here I want to use it!

How would you deal with this, as I have only been programming professionally for 4 months!