
There hasn’t been a lot of Daryl Dixon in the second half of the season but he is still a major topic of discussion, or rather his relapse is a major topic of discussion. He seems to have reverted to the Daryl we first met, all that progress in the first half of the season appears to have been undone. I don’t think it has I think what you are seeing is the Dixon Method of grieving.
Daryl invested a lot into finding Sophia safe and sound and although Sophia was found, she was anything but safe. She was a walker, that had to be a major blow for him. And as we all should know by now, Daryl is not the most emotionally mature person who has survived. He has a temper and his usual emotional response is to lash out at others.
I tend to refer to finding Sophia as Daryl’s ‘Quest for the Holy Grail’. We hoped it would end well but are we really surprised it didn’t? And being that it didn’t are we really surprised that Daryl is not taking it well? Sophia was a symbol of hope for him.
I think to really understand his reaction you have to think about why he was searching so hard for her in the first place. After all everyone else gave up hope including her own mother, but Daryl clung to that hope like a lifeline.I personally think it is a combination of three reasons and not a single one of them was because his “other plans fell through” as he told Rick.
Reason 1-It gave him a sense of worth within the group.

Let’s face it, no one lost any sleep over the fact that Merle up and disappeared and everyone’s opinion of Merle is very clear. Shane calls him a douchebag and no one argues with it other than Daryl. Daryl is Merle’s baby brother and from the conversation around the campfire before Daryl returns it’s also clear that their opinion of Daryl isn’t much better. Finding Sophia was a chance to show people that he was not just a younger version of Merle, but his own man.
Reason 2-If he can find Sophia, maybe he can find Merle too

Merle is not dead, like Sophia he is missing. Finding Sophia meant he could hold on to the hope that his brother may still be found alive. While everyone else may call Merle douchebag, Daryl calls him big brother. Merle is his family and the only family he has left. Family tends to get unconditional love. Merle may not have been the best brother in the world, but he is the only brother Daryl has and having Merle around would mean that Daryl is not ‘alone’ anymore.
Reason 3- No one looked for him

Although when he tells Andrea about the time he got lost he makes it sound like no big deal, he was younger than the 12 year old Sophia. A young child getting lost is frightening and knowing that no one really cares that you are lost is even worse. It will leave a scar even if it is not visible. I’m sure Daryl probably had a few nightmares about it as a kid and he didn’t want Sophia to go through the same pain.

Although I love character of Daryl to pieces, he does have his flaws and his emotional immaturity is probably the biggest one of all. He can grasp things mentally but he is not prepared to deal with them on an emotional level. He sees things differently than the others and probably often sees innocent comments as attempts to push him away. Rick has done this more than once. Rick will say something and then add “I’m letting you off the hook.” but that is not what Daryl is hearing emotionally. What he is hearing is “we don’t need or want you.”
Yes there are some issues that Daryl needs to work through, but he is going to have to learn how to better deal with those issues. The group is also going to have to realize that he is seeing things differently than they are and realize that he needs reassurance of his place from everyone not just Carol.