I don't believe in 'random karma'*, but I do think there is something to be said for 'logical karma'*.
For example, lets say a student bullies another classmate at school. 'Random karma' would be that they get the flu, but 'logical karma' would be that they lose friends. See, to me, 'random karma' doesn't make sense - they're mean, so they get the flu as punishment? - but 'logical karma' does - they're a bully, and as people realise this, they will not like the person and will stop being friends with them.
If you do good things, people will like you, you'll see the world in a more positive way, and be happier. If you do bad things, people won't like you, you'll see the world in a more negative way, and be unhappy.
Basically: Your actions have consequences.
*These are terms that I have made up in an attempt to explain my point, they aren't related to any religion or other explanation of karma (at least, not that I'm aware of)