Going to the gym may make you less likely to perform a favour.

Feelings of guilt are important drivers of prosocial behaviour. If we feel guilty about something, we are more likely to do a favour for someone else.
Ego depletion is a state in which our combined resource for emotional, physical and mental effort has been drained by exertion in one of those areas, and has not had time to recover.
If someone is in a state of ego depletion they are less able to feel guilty and are therefore less likely to participate in prosocial behaviour – that is, behaviour which helps other people or society in general. Therefore, if you have had a hard session in the gym you are likely to be ego-depleted and are therefore less likely to perform a favour for someone else.
Begue and Bushman tested this hypothesis by getting their participants to watch a horrible film, but half of them had to suppress their emotions (one way of producing a state of ego-depletion). All the participants then took part in a game in which they saw other people being punished for the participants’ errors. This was designed to induce feelings of guilt. Finally, the participants were given the opportunity to donate, anonymously, to a charity. The people who had been told to suppress their emotions felt less guilty than the others, and were less likely to donate to the charity.