Short Answer
Potential reasons include air leakage, water intrusion, rot, damaged frame, poor operation, security concerns, deteriorated weatherstripping, and outdated performance.
Detailed Explanation
Exterior doors should be evaluated for both performance and condition. Signs that replacement may be warranted include: drafts that can't be corrected with new weatherstripping, water intrusion during rain (indicating flashing or threshold failure), rot in wood doors or frames, doors that don't operate smoothly or don't latch properly, security concerns (old or weak locks, thin doors), deteriorated or missing weatherstripping, and doors that are single-pane glass or poorly insulated. Like windows, some door problems (weatherstripping, hardware, threshold seals) can be repaired, while others (rotted frame, structural damage) typically require replacement.
Why It Matters
A failing exterior door wastes energy, allows water intrusion, and compromises security. Knowing when to repair vs. replace prevents both unnecessary expense and ongoing problems.
When Repair Makes Sense
Problems are limited to weatherstripping, hardware, or threshold seals that can be replaced.
When Replacement Makes Sense
The door or frame is rotted, structurally damaged, or poorly insulated, and repair won't restore proper performance.