Situations that especially call for one
A second opinion is particularly worth getting before any large or expensive treatment plan, before agreeing to an irreversible procedure like an extraction, before starting implant or full-arch work, or any time the reasoning behind a recommendation isn't clear to you even after asking directly. It's also reasonable if a recommended treatment plan feels significantly larger or more expensive than you expected going in, or if you're being asked to decide quickly without time to think it over and compare it against another perspective from someone who has no financial stake in whichever way you ultimately decide to go — pausing to think costs nothing at all when the situation genuinely isn't a true dental emergency. Taking that extra bit of time rarely changes the underlying problem, but it can change how confident you feel about the plan you eventually choose.