I won the match because I practiced for 100 long hours, but if I lost, it was due to the umpire's bad decision or wind speed or bad equipment.

Individuals justify success with their work, strengths, responsibilities, but believe that the cause for their faults and failures were external factors.

For wins, individuals reward themselves and for the loses they criticize other situational factors.

Why?

Ego and Self-esteem.

P.S : When any failure occurs, ask yourself "Was it because of the situational factors or because of my own?"

References

1. Self-Serving Bias : sketchplanations
2. International Encyclopedia of the Social Sciences, 2nd Edition