sunsheen

Rick was most likely upset that Morty was developing a new love interest that was healthy (at first) for two reasons:

1) He's afraid of Morty becoming independent and leaving him

2) He's upset that Morty is finding love when it worked out badly for Rick in the past

Those two reasons are also why he decided Summer would be the perfect person to go on an adventure with:

1) Rick wants Morty to believe that he's replaceable and hopes that favouring Summer would make him jealous

2) Summer had gone through a recent breakup and therefore had a negative outlook on the concept of love

Clearly, regarding the latter point, Rick is being manipulative in the sense that he's using Summer's sadness to get back at Morty *and* feel better about his own negative emotions. The same situation of Rick bringing Summer along because Morty refuses to follow Rick is shown in Season 1 Episode 7, which, mind you, he consistently belittles her for being a female.

me: *knows that I don't do well under a tremendous work load* ok that's cool but it's my life and I'd kinda rather go to the University of Sheff-

my friend after accidentally calling me "her" yesterday: ....just gotta check with Liss to make sure it's ok with them