You have to lift up your body mass under the presence of gravity, so you have to overcome the force of gravity. While climbing up the stairs you have to put force on ground, more then your weight, which put same force but in opposite direction i.e. on you.
Answers & Comments
Verified answer
why is it difficult to climb up the stairs rather than go down the stairs? you go with gravity
ANSWER:
B. Are against gravity
You have to lift up your body mass under the presence of gravity, so you have to overcome the force of gravity. While climbing up the stairs you have to put force on ground, more then your weight, which put same force but in opposite direction i.e. on you.