There are many reason you could miss a flight.
You must be in your seat 15 minutes before schedule departure time.
Mistakes I see daily include:
Checking in late, you must allow a least 1 hour before departure to check in and check bags. There is a bag cut-off times in every airport. If you do not get to a customer service agent before that time, you miss your flight.
Getting through security. On a busy day it could take you over an hour or more to get through. Take into account time of year, Holiday, Summer, Spring Break, all extremely high volume times.
Parking. In large airports there may be many options to park. Many include taking a bus to the airport terminal. It takes time.
Document check. If you are leaving the USA, your documents have to be verified. You must have what you need according to the country you are traveling. If not, you will be denied boarding.
Misconnection due to late inbound flight.
The airline should rebook you to the next available flight.
There are many reason the airlines could be delayed. If you have to spend the night due to a misconnect the airline should pay for your hotel if the reason was a mechanical or crew delay.
Other reason will be up to the traveler. If it was due to weather, local, enroute or destination. ATC, or Security.
I will discuss next time what will happen when you miss a flight.
Until then.....do not be late to the gate!
I Fly Right