"Wishing you happy birthday would never come close to all the inspiring things I wish for your life," Valderrama, 34, wrote alongside a pic of the two wearing matching holiday-pattern hoodies. The adorable snap shows birthday girl Lovato, 22, looking up at Valderrama and laughing.
"We laugh our asses off," Valderrama continued. "While we do all we can to better everything around us.. Love you through all."
Valderrama's birthday greeting was similar to Lovato's b-day message for him back in January, in which she described him as "the funniest, most inspiring, incredible man I know." Along with thanking her beau for helping her endure tough times, Lovato added: "You truly are a light to this world and to my life."
The "Heart Attack" songstress and That '70s Show alum — who's dated Lindsay Lohan and Mandy Moore — first hooked up in 2010. They split and started seeing each other again the following year, while remaining secretive about the relationship.
In recent months, the two have become significantly more public about their romance. Valderrama recently made a surprise appearance in Lovato's "Really Don't Care" music video, which features the couple cozying up to each other at the L.A. Gay Pride Parade in West Hollywood.
Lovato, on her end, expressed to Cosmo on Campus back in February 2013 that she was looking for a romance. "I'm in a place where I can finally trust and love and be a better person for the man I am in a relationship with, instead of being a clingy girlfriend or getting jealous," she said. "It is very unhealthy when girls devote all of their time to a guy and forget their friends and family. I did that. I was always looking for distractions because I was so afraid of being alone."