Events in life result because of deterministic and non-deterministic sequence of actions. Taking credit for deterministic actions is an easy thing and hence not really interesting. Here I am talking about personal credit, not credit given by society because of being the medium for events. According to karma we can take credit for non-deterministic actions but it is usually hard for most to be good accountants. It takes experience, courage and many more hard qualities which grow by practice to be good accountant.
A subtle but important point to realize is that we are only playing a role of verifier in the system (Nature if you may think so) where some sequence of actions result in events. Since we do not explicitly know who plays the role of prover we cannot take "complete" credit (personal) for events even if we believe in karma. Believing in G-d (I am not talking about belief in after-life or being religious etc., which are only some efficient tools) is probably the optimal choice for the role of prover, who plays a key role in the events.
One important point is that we need to assume the prover and verifier are both honest for the completeness requirement of the system. It is necessary that the verifier is honest for the system to be sound. One can imagine that if there were an all-powerful bad-guy then honest verifier will not allow him win!