The formula for average speed is:

total distance = (average speed)(total time)

When you travel the same distance at two different speeds, you spend more time at the slower speed (because you are traveling slower and it takes more time to travel that same distance). This means that the value of the average speed for the entire trip will be closer to the slower speed since you spend more time at that slower speed.

As an example, if you travel 120 miles at 60 miles per hour and then travel 120 miles at 40 miles per hour, then that is a total of 120 miles + 120 miles = 240 miles and 2 hours + 3 hours = 5 hours.

(120 miles+120 miles) = (average speed)(2 hours+3 hours)
240 = (average speed)(5)
240/5 = 48 miles per hour = average speed