karmanaha sukritasyaahuhu saattvikam nirmalam phalam |
rajasastu phalam duhkhamajnyaanam tamasaha phalam || 16 ||

The result of good action is sattvik and pure, it is said, while the result of rajas is sorrow, and the result of tamas is ignorance.
karmanaha : action
sukritasya : good
aahuhu : is said
saattvikam : sattvik
nirmalam : pure
phalam : result
rajasaha : rajas
tu : while
phalam : result
duhkham : sorrow
ajnyaanam : ignorance
tamasaha : tamas
phalam : result
When new year comes around, many people make new year resolutions to lose weight. Many take a gym membership, but most end up using it only for a few days. The ones that consciously develop a habit of going to the gym everyday are the most likely to lose weight. Even if they go to the gym for ten minutes every day, they will lose weight. This is because habits are self reinforcing. A behaviour creates a habit, which pushes us to repeat the behaviour, which strengthens the habit, and so on.
Similarly, we can shape our mental states by choosing our actions consciously. Shri Krishna says that sattvic or selfless actions generate a sattvic state, rajasic or selfish actions generate a rajasic state, and tamasic or ignorant actions generate a tamasic state. If we are predominantly rajasic by nature, we can improve our state by emphasizing the performance of sattvic actions. If we are predominantly tamasic by nature, we can change it by emphasizing the performance of rajasic actions.
The law of karma holds true in every situation, including this one. If we perform rajasic actions, we will attain temporary joy, which will eventually generate sorrow. It does not take a large amount of raaga dvesha, likes or dislikes, to make us act selfishly. And if we perform tamasic actions, we will continue to live our lives in inertness and ignorance. Only through sattvic actions will we create a state of purity and serenity in our mind.