The Republic of Ireland international netted twice, along with Brazilian midfielder Juninho, as the reigning MLS champions beat Toronto FC 4-2 on the weekend and Arena was thrilled to see him net his 13th and 14th goals for the season.
"Well, obviously, Robbie is here to score goals, and he's done that," said Arena.
"Robbie continues to get chances. Good to see him finish two tonight. I just think we're moving forward as a team, you know."
The Galaxy have won seven of their last nine in both the MLS and CONCACAF Champions League and Arena is proud of his players for the form they have shown of late.
"The resurgence has been outstanding; it's a real character test on the group. Not many teams would have positioned themselves where we are right now," he added.
"Having said that, I think we all know there's a lot of soccer left to be played this season and we really got to be determined over the next four games."