Why do targaryens marry each other?

Related questions

1. Can only targaryens ride a dragon?
2. Was Balerion the biggest dragon ever?
3. How did khaleesi get dragons?
4. What are the names of Daenerys dragons?
5. Why are the targaryens called dragons?
6. Can khaleesi control her dragons?
7. What happens to Khaleesi's Dragons?
8. Do bearded dragons know their owner?
9. What do you call a baby dragon?
10. Can there be more dragons in Game of Thrones?

Tags

1. #children
2. #siblings
3. #bloodline
4. #cousins
5. #targaryens
6. #daenerys
7. #marry
8. #least
9. #often
10. #first
11. #three
12. #products
13. #other
14. #sisters
15. #possible
16. #incest
17. #their
18. #rhaegar
19. #viserys
20. #would