Trying new things is how people advance professionally. It's how you increase your skills and show that you can stretch beyond where you currently are. It's also how people get raises – not at the outset, when you're first taking on the new work, but later, after you've shown that you can do it well. If you reflexively get put out when someone gives you a new task, you're likely to kill your growth potential, and your boss may start wondering whether you want the job at all.
(To be clear, there are times when it's reasonable to raise concerns with your manager about adding to an already large workload. And if the new work causes a real hardship, such as constant travel, that's reasonable to speak up about.)