The 48-year-old actor and 51-year-old triple-threat star were engaged from 2002 to 2004 and a source tells ET that they have "picked up where they left off after all these years."
"Although they planned to take things slow, their connection has made it hard," adds the source. "Jennifer and Ben are very into one another and in a short time are very serious."
As for how the two are making their relationship work while also tending to their children and professional obligations, the source notes, "Jennifer is spending her time in Los Angeles and so are her kids. Ben and her kids have spent time together and everything has been great."
Things sure do look great between Affleck and Lopez, who were spotted kissing in Malibu, California, while out to dinner with Lopez and ex Marc Anthony's 13-year-old twins, Max and Emme, in celebration of her sister, Lynda Lopez's, birthday.
"Everyone feels really comfortable around Ben and likes him a lot," a source told ET earlier this week. "They don't have one bad thing to say about him. They think he's funny and sweet and it's so nice for all of them to see Jen so happy and smitten. Ben definitely has their stamp of approval."