"I'm happy." The first time we hear those words from Oliver Queen, he's in a car with Felicity Smoak leaving Star City. He's talked about being happy because he's with Felicity but never when he's alone. Let's face it. How many of us are happy completely alone?
Oliver endured five years of hell we've only seen glimpses of in flashbacks, and he came back to Star City a changed man, a damaged man. Though friends can help heal wounded spirits, oftentimes it's love that mends broken souls. He needs someone who is on his side, regardless of the mistakes he might make in the future.
Oliver was tortured, forced to kill, watched his father commit suicide, was unable to keep his best friend from dying, witnessed his mother's brutal murder, and almost lost his sister. If anyone needs love, it's him. Quite honestly, I think it would be heartless of the writers to leave him alone, even though watching a dark, brooding Oliver can be enjoyable.
Though Oliver has a solid friendship with Diggle, he still needs a soft place to land when the world beats him down. He needs someone to be in his corner, someone to pull him back from the edge, to stand next to him when he has to take the next leap, and to love him unconditionally. That's what will make the dangerous life he leads a little more tolerable.