Smolder is a dragon, she will live a lot longer than her friend so she can take her time deciding what she want to do with her life. Dragon barely have a society and mostly live in the wild so it make sense she would just hang around with her friend, even as an adult.  
I like the idea of her and Spike being close but not intimate. Smolder somehow became a bridge between Spike and dragon’s culture in general.  
I said she wanted to be an adventurer because of the compass she had in the last episode.
 
Also, isn’t it weird she barely changed in comparison of Spike ?