A filling would have nothing reliable to hold on to.
A crack that needs binding
Full coverage resists flexing.
After root canal on a back tooth
Load and brittleness combine.
A cusp has already fractured
A pattern that tends to repeat.
A large restoration is failing repeatedly
Diminishing returns on patching.
When it may not be
A moderate cavity with strong walls
A filling or onlay may serve.
An old but sound filling
Age alone is not an indication.
A cosmetic concern on a front tooth
A veneer or bonding may be more conservative.
A hairline craze line
Usually needs nothing.
What I would ask
If a crown is proposed, ask what specifically makes a filling or onlay insufficient here, and what happens if you choose the more conservative option. Both questions have real answers, and hearing them is how you judge the recommendation.
A second opinion is reasonable for any large plan. It is not an accusation, and no dentist worth seeing takes it personally.
What we can explain here — and what needs an exam
A page can inform you. It cannot examine you.
We can explain online
The structural reasoning behind crowns
More conservative alternatives
What questions settle it
Determined after an exam
How much sound structure your tooth has
Whether a crack is present
Which option is right for you
What to do next
Ask the structural question. If the answer is clear, the recommendation is probably sound.