This purely my opinion, so don't call me meanie or a sexist or whatever.
Season 1: We first see Andrea as a supply runner along with T-Dog, Merle, Glenn, Jacqui and Morales where he pushes a gun in Rick's face for using his. When I first saw Holden, I thought she would be an original character because surely, this woman was not a character that I would have known! But no, she turned out to be Andrea, the sharpshooter we all grew to appreciate. After returning back to the Atlanta camp, she goes fishing with her sister and talks about vibrators. After the camp gets invaded, she has to put her sister down and before doing so, she tells complete strangers about how she could never make it to any of Amy's birthday parties. After realizing that they should leave, the group packs up and heads back into Atlanta to go to the CDC, once there, they enjoy air conditioning, food, running water, and a good night's rest. But when it was revealed that the CDC would blow up, Andrea decided to stay because she was, understandably, distraught. And thanks to the wisdom of Dale, she changed her mind and they met up with the rest of the group.
Why it happened: Andrea's actress, Laurie Holden has worked with Darabont, the original showrunner before. And he made a very biased choice by casting his friend for a role she had no business playing. My only real Andrea complaint for season 1 is the fact that she was casted. She's a fine actress but she's too old for Andrea and she just didn't fit the role Andrea had in the group.
Season 2: This is where season 2 gets rough. So while the group hides from a horde, Andrea fiddling with her gun when she realizes what happened and runs into the bathroom. A zombie, somehow smart enough to figure to walk into the RV, hears Andrea and attacks. Dale gives her his screwdriver and she kills the walker after much struggle. This proves the show just wanted a melee kill because no other member of the undead walked into the RV or even heard the ruckus that was happening. Later on after reaching a farm, she wants to prove herself to DA GUIS bye shooting one POSSIBLE member of the undead despite being told not to AND after four perfectly capable men go out to check to see if it IS a walker. But no, to show off her #GURLPOWER, she shoots the figure anyway, who ends up being a, bruised, beaten, dirty, and tired Daryl. One thing that really grinded my gears is how she didn't even try to talk Beth out of her suicidal 'phase' for lack of a better term. Instead, she just leaves her be, assuring everyone that she won't do it. Even though she didn't kill herself, you don't talk to a suicidal person like that. Later on, after the farm gets overrun, she escapes to the forest and hours later, gets saved by a mysterious stranger with a sword!
Why it happened: I understand that they wanted to show that Amy's death affected Andrea and all but she seemed completely oblivious to her surroundings on the RV. I'm sure she would have noticed the undead walking past her sooner than she had. And she treated Dale so horribly for saving her life. If she wanted to kill herself so bad, there was always a gun around. Just saying. And shooting Daryl was just silly. And then, her whole suicide adventure was stupid beyond rules.
Season 3: After traveling for months with a woman named Michonne, she becomes sickly ill. Michonne gets medicine for Andrea. Andrea gets better. Michonne and Andrea find a town called Woodbury ran by a man who called himself the Governor. Andrea sides with a man whom she had known for about 3 days rather than the person who had helped her survive for months. Andrea sleeps with the Governor. Andrea realize Governor BAD then goes to the Prison where he old group is at. She had the chance to kill the Governor while he couldn't defend himself but chose not to. And a bit before, she even say the Governor's zombie head collection but decided to stay instead of going with her friend. Then, she actually gets killed because of the Governor. #Irony #Twitteronwikia #amcsthewalkingdead
Why it happened: Mazarra and his band of shatty writers. ;)