It's a combination of things, I didn't like how Prof. Oak was put in a situation because he likes both of them (although I think he favoured Gary, being his grandson of course, on a few occasions).
The other is Gary was mean and obnoxious to Ash on countless occasions, it might (if Ash wasn't so stubborn

) have hurt Ash on some of those occasions which is why he fights so hard to win, acted the way he did in S1 and during the Johto region until the episodes during the League. (especially as now we have Gary asking Tracey and Prof. Oak what Ash is up to...)
As for the episode, I thought it was really good, I like how Delia knew that her son was going on another adventure, I mean she didn't want him to go but she didn't mention it to him