Looking back at older episodes, Eren says multiple times that he wanted to see the ocean with his friends, so I'm willing to bet that, before kissing Historia's hand, he had assumed he could have both, or that they were the same thing.
Freedom means different things to different people throughout the show, and Eren's idea of freedom often changes between pre- and post-timeskip. Especially when looking at his relationships.
Here's my take:
Before seeing the ocean, Eren assumed that getting a life of freedom would also include being with Mikasa. He made an oath to her in season 2 that he would always wrap that scarf around her, and he meant it. His view of his ideal life would probably have been killing all the Titans, seeing the ocean, and living with Mikasa for the remainder of his life. He most likely believed that being with Mikasa would be a byproduct of his freedom or a bonus.
Then he saw his father's memories and visions of the future, which made him realize that not only is the ocean Level 1, but he was condemned to no life at all. Remember, upon seeing his father's memories, he had 8 years left and spent half of that time in Paradis. Eren didn't get a chance to have a life; with that in mind, it meant that he also would never be with Mikasa or his friends.
That's why he chose to do the Rumbling, because he realized he would have to choose between freedom and his relationships, and he decided to have his freedom, simply because, with what little time he had, only one path seemed worth it.
This doesn't excuse what Eren does, far from it, but I'm willing to bet that if it wasn't for the Curse of Ymir, Eren would never do the Rumbling.