1. Your bones are extremely dense, receive 5 extra HP's.
2. You are exceptionally large for your species, so you receive 5 bonus HP and are one size category larger than is typical for your race.
3. Your naturally good at dodging and rolling with blows, receive 5 extra HP.
4. You receive the feat, "Extremely Tough", gain 5 bonus HP's.
5. You unknowingly drank from a magical pool, permanently receive 5 bonus HP's.
6. A meteor landed near you when you were younger. The magical radiations gave you 5 bonus HP.
7. That potion you drank was actually bad, however its random effect gave you 5 bonus HP.
8. Your momma slapped you around a lot as a child, as a result your much more capable of dealing with pain, receive 5 bonus HP to reflect this.
9. You were caught out in a magical storm that was the direct magical manifestation of a godly argument. That lightning bolt nearly killed you, and would have, but at the time it struck you it gave you 5 permanent bonus HP.
10. You regularly ate your spinach as a child, receive 5 extra HP, permanently.
11. Your family bought some "mystery meat" cheap at the local market. You received 5 bonus HP after recovering from the illness it caused. Unfortunately you are the only member of your family that made their CON save versus poison. On the good side, you inherited everything.
12. Your father was a Ranger, and he regularly mixed Troll blood into your food. As a result you gain 5 bonus HP.
13. Your mother was a druid who killed a young dragon rampaging through her forest. She made a herbal concoction that was "good for you". After several attempts you successfully choked it down. Soon after you felt better than ever. 5 bonus HP received.
14. Your father was a wizard who performed many experiments upon you. You have hundreds of scars, many of them nasty looking, but you also have 5 bonus HP.
15. Your father is a priest of the God of Strength. After many years of rituals performed upon you the god has permanently blessed you with 5 bonus HP.
16. As a teenager you finally found the party the Pixies were rumored to throw on the full moon. Your "mystical" experience left you with 5 bonus HP.
17. A "Seller of Magical Elixirs" came through, successfully selling you a "Elixir of Manliness", it didn't work in the manner you were told it would, but it did give you 5 bonus HP.
18. You found an incredibly large egg in your hen house. After eating it you felt noticeably healthier than ever. It gave you 5 bonus HP.
19. You had "THE WICKED STEP MOTHER" and survived her many attempts to make you go away, permanently. You received 5 bonus HP as a result.
20. You sneaked up on a Unicorn in the forest. Then it noticed you it kicked you in the head. When you woke up you felt healthier than ever. You received 5 extra HP.