Most symptoms of the “milder” mental disorders (depression, anxiety, adhd, etc.) are things that everyone experiences at some point in their lives.
It’s the degree of experience, frequency, and impact that differentiate the two. Also, when those symptoms don’t have an environmental/situational/logical cause.
For example, being too anxious to go out with friends, or compulsively checking that the stove is off even though you know damn well it hasn’t been turned on in a month. Also, being sad because your pet died isn’t a mental disorder like clinical depression.
The line between the two is definitely fuzzy, but psychology is a super complex topic.