Why can't Git merge file changes with a modified parent/master?

Question

I have a file with one line in it. I create a branch and add a second line to the same file. Save and commit to the branch. I switch back to the master. And add a different, second line to the file. Save and commit to the master. So there's now 3 unique lines in total.

If I now try and merge the branch back to the master, it suffers a merge conflict.

Why cant Git simple merge each line, one after the other?

My attempt at merge behaves something like this:

PS D:\dev\testing\test1> git merge newbranch
Auto-merging hello.txt
CONFLICT (content): Merge conflict in hello.txt
Automatic merge failed; fix conflicts and then commit the result.
PS D:\dev\testing\test1> git diff
diff --cc hello.txt
index 726eeaf,e48d31a..0000000
--- a/hello.txt
+++ b/hello.txt
@@@ -1,2 -1,2 +1,6 @@@
  This is the first line.
- New line added by master.
 -Added a line in newbranch.
++<<<<<<< HEAD
++New line added by master.
++=======
++Added a line in newbranch.
++>>>>>>> newbranch

Is there a way to make it slot lines in automatically, one after the other?

Answer 1

Let's say the file branch A looks like:

First line
Branch A's second line

And branch B looks like:

First line
Branch B's second line

When you merge, there are two different ways to resolve it. Here's one way:

First line
Branch A's second line
Branch B's second line

Here's another way:

First line
Branch B's second line
Branch A's second line

Git has no idea which one of those two options you prefer, or if either are even acceptable merges.