Measuring Speed

I can measure the speed of an object, because I want to km/s.

Speed
Object
Depression
Distance
Measure
Vehicle

Explanation

The humor comes from the unexpected way the speaker reveals their desire to feel depressed. Instead of saying they want to measure the speed of an object in km/h or m/s, they say they want "km/s", creating a nonsensical measurement unit which can be read as "kill myself".