Back in Pokemon Yellow, there was a glitch with the potion and Pikachu:
1. Take potion from PC.
2. Receive Pikachu.
3. Make sure Pikachu has full HP.
4. Use potion.
5. Spam it.
6. Eventually, Pikachu's happiness will be 100.
Now, does this still work in other Generations, or am I wasting my time with Pichu?