NB: Edited to fix a few typos and grammatical errors.
1. Depression is not an emotion (i.e. "sad") and cannot be cured and/or counteracted by sufficient forced exposure to "happy" or "love." Episodes often have no identifiable causes for onset or remission. Do you blame people with epilepsy and their families for causing the seizures? People with depression and their families have the same level of responsibility.
2. Major Depression (recurring) is a physical illness outside the control of the conscious mind. Just like the bodies of diabetics cannot properly regulate insulin and blood sugar levels in response to eating, the bodies of depressives cannot properly regulate the brain's levels of neurotransmitters in response to sensory stimuli. Also like diabetics, depressives can change some habits and behaviors to mitigate their symptoms and live "normal" lives much of the time, but understand that in emergency situations, medical intervention is necessary. In neither case does will power, level of personal discipline, and/or perceived strengths or weaknesses of character have any effect on the disease itself. In both cases, denial and refusal to accept/seek/continue treatment can be deadly.
3. Cultural stigma surrounding depression causes many people experiencing depression to self-medicate before/instead of seeking treatment, putting themselves at risk of overdose and/or aggravating their illness. Even if they are lucky enough to find a doctor and treatment that helps them function independently again, people with recurring Major Depression are often pressured by peers and loved ones to "finally get off the pills" and "(exercise/smoke pot/find religion/etc.)". See above for how denial and refusing treatment usually works out.
4. Are you a trained psychiatrist/psychologist/social worker? No? Then it's not your job to try to cure or fix someone with depression. It's your job to be their friend/family member/colleague. Focus on that job. If they're acting in an alarming manner, please call in the professionals. Otherwise, you're the part of their life that *isn't* their illness: i.e. the part of life that they still love. If they're clinging to the ledge of the abyss, giving them a hand to hold onto, reminding them you love them, listening to whatever they feel the need to say, or just doing little things like holding up a straw so they can sip from a smoothie keeps them from falling before the Medi-vac arrives, and also helps distract them from the terror of dangling over the abyss alone.
5. Depression is not only a real, chronic illness, it's often a terminal one. Living with Major Depressive Disorder requires people to win a staring contest with the abyss every day. Yes, loved ones can help and cheer, apply some Visine if you've got it on hand, but ultimately the contest is one-on-one. Eventually a person will blink. It sucks, but it's not your fault. The game is rigged. Blaming yourself, or them, for giving in to that need won't help you heal. It's the memories of all the victories you celebrated together which will discourage the abyss from challenging you to the next match.