clipped from: allpsych.com   
Many factors play a role in how we assign attributes to behaviors.
1. Fundamental Attribution Error.  This refers to the tendency to over estimate the internal and underestimate the external factors when explaining the behaviors of others.  This may be a result of our tendency to pay more attention to the situation rather than to the individual (Heider, 1958) and is especially true when we know little about the other person.
2. Self-Serving Bias.  We tend to equate successes to internal and failures to external attributes (Miller & Ross, 1975).