Well, first of all, tehre are a lot more other competent people can rule a country aside from the new generations of other rulers.  Second, political dynasty gives too much power to the family involved.  Next, it usually makes the rulers' family "addicted" tot he power and glory that it gives.  Lastly, corruption becomes much more easier for the family that rules.

We are all entitled to our own opinions.  This is just my perspective on the given topic.  Nonetheless, hope this helps :))

p.s. If you do not understand a part of my solution, please do not hesitate to contact me :)
Because some of them becomes an opportunist and greedy in power.