Help: players have lost sense of danger
5.5E DND. I've been fairly generous with cool items and my players are overall pretty good at strategizing and utilizing their abilities during combat, including those for whom this is their first campaign. On one hand this is great because I can throw powerful enemies at them without worrying about them dying. On the other hand, they've kinda lost all sense of danger and I could literally introduce a tarrasque in the middle of the fight and they'd go "sure we can take it". I want some encounters to be fairly easy but I want the tough ones to give them a feeling of "I don't know if we can survive this" followed by the satisfaction of surviving it. How would you go about reinstating a sense of danger to your players? Should I just design tougher encounters until they realise that death is on the table, or is there a smarter approach I could use? Or is it just a non issue since everyone at the table is having fun anyway?