Lebron wants to win as many rings as he possibly can - all he cares about is cementing his legacy and trying to overcome Jordan.
Look at Lebron's history. He tends to go to teams where he can join up with young stars so that he can compete for many years. Joined with Wade and Bosh in Miami when they were in their 20's Once he felt they no longer had potential to compete in the future, he walked.
Next he went to Cleveland to join up with two younger stars in Kyrie and Kevin Love - both in their low-to-mid 20's. Had a solid run, but now Cleveland are clearly outgunned and outmatched, and they lacked the cap space to sign free agents or any high picks to draft a future star.
Gong to the Lakers, he could join forces with Ball, Ingram, Randle and Lopez - given how dominant Lebron is (every team he touches turns in to a contender) that core is probably good enough to make the east semi finals at the very least. Then next year they have tons of cap space, so they likely sign Paul George (everybody knows he wants to go there). Now you have Ball, George, Ingram, Randle, Lebron, Lopez, Clarkson and more - that's probably a better team then he had in Cleveland last season and can likely challenge golden state...but the fact that they have those young studs young studs means that as Lopez and Lebron start to decline, those young guys can develop and do more of the heavy lifting - maximising Lebron's championship window. He could potentially still be competing for titles at 38 years of age as a supporting piece to the Laker's new big 3 of Ball/George/Ingram, well after Golden State's rein in the West has ended.
Now if Thomas can eventually return to last season's from before the end of the year, then the fact that the Cavs added Thomas, plus Crowder (on a cheap long term deal), plus Zizic plus the Brk pick may force him to reconsider and sign for a couple more years.