tl;dr Ego's main objective is to carry out his plan, his "purpose", everything else was secondary and a distraction to him.
Ego is the Sailor
In the scene just after Gamora has a small argument with Peter, and she storms out, Peter is lying down on the couch and listening to Brandy by Looking Glass
Ego enters and exclaims to Peter that this is
one of Earth's greatest musical compositions, if not the greatest
-Guardians of the Galaxy: Vol. 2. [paraphrased from memory]
And sings along to certain parts of it:
He [the sailor] came on a summer's day
Bringing gifts from far away
But he made it clear he couldn't stay
No harbor was his home.
Brandy, you're a fine girl
What a good wife you would be
Your eyes could steal a sailor
From the sea.
He then comments to Peter:
You know Peter, we are the Sailor
This is where it starts to unravel, by saying that he is the sailor, Meredith is essentially "Brandy" and the "sea" is the Cosmos and his "purpose" within it.
But my life, my lover, my lady is the sea
Given all of this, let's have a look at the rest of the lyrics to the song:
Brandy, you're a fine girl,
What a good wife you would be
But my life, my love and my lady is the sea.
Ego tells Peter that he first encountered love when he met Meredith, and I believe him. He truly loved Meredith:
My Riverlily who knew all the words to every song that came on the radio
And he truly did consider making her his wife. But unfortunately, he felt that his connection to her was going to be distracting from his true purpose, his seeding and re-aligning of the universe.
He says clearly that his attraction to her was pulling him away from his mission and he couldn't bear to be around her (which I assume applies to whether they're on Earth or on his planet). So he
planted that tumour in her head
Logistics man, logistics
I have my own understanding for the reasons that Ego may not have taken Meredith to his planet (or anywhere else with him):
- What if the child didn't carry the Celestial genes?
Ego is shown to have clearly disposed of his "failed" offspring. Was he going to do that in front of Meredith if Peter didn't have the genes?
- What if the child did carry the Celestial genes?
Once Ego realised that Peter did carry the Celestial genes needed to help him power his universe-overtaking-smog-seed-thing, he enacted this plan which involved him basically harvesting Peter's energy force. Would he have done that in front of Meredith?
Meredith belonged to Earth
It made her, as she made it. Ego fell in love or whatever he felt with Meredith on Earth; his whole experience with the driving around, listening to music and whatever other 80s things they did, was what made him feel that connection to her. In fact, he even said that he
couldn't bear to visit an Earth where Meredith Quill didn't exist
He simply may not have loved or cared for her anymore if the experience had changed; then what, dump her anyway?
He is a god, these things are beneath him
Ego is (get ready for it) egotistical. When Peter questioned him about his "friends", Ego responded:
we're gods Peter, these things are beneath us
Clearly showing that he held no regard for long-term sustained relationships, as they would eventually fade away. Hence, when it came down to choosing Meredith Quill, or fulfilling his purpose, he chose his purpose.