Leather sofa's can be very durable, but the stitching is often the most vulnerable. The stitching on our 10+ year old IKEA sofa was starting to come loose in one section, so I went to work repairing it, and here's how>
For starters, you'll probably want to do any repairs as soon as you see any separation.
You'll need an appropriate color of upholstery thread and a sturdy hand quilting needle. Let's begin!

Step 1: Vacuum and Remove the Cover.

When was the last time you vacuumed in between the cushions of your sofa? Here's a great opportunity, and it will make the rest of this job much less gross! My thrifty IKEA sofa has a zipper to remove the cushion, so I am going to assume that most leather sofa's have removable covers. If not, these instructions won't be as helpful. Remove your cover, and identify and mark the back side of the repair location.

Step 2: Identify the Type of Repair

Most likely your leather or vinyl upholstered furniture is assembled in this manner:
Adjacent Panels are first aligned and stacked together, face to face and stitched along required edges. The seam is flattened and a fabric tape is stitched through all layers of each side of the seam. If this secondary stitching is coming loose, then you can use either a sewing machine or hand sew overtop of the section that needs to be repaired. That repair is fairly straight forward and easier to accomplish.

If your loose stitches are like mine (the initial stitch), it's a little trickier to repair as the fabric tape is blocking access to the area that requires repair. You could remove the tape temporarily, but you risk causing more repair issues that way, and it's better not to punch a bunch of additional holes in your leather.

Step 3: Sew Things Up!

Lift the edge of the seam tape and guide your needle straight under and through to the same area of the connected panel. The photos help explain this more clearly. After your first completed stitch tie a couple of secure knots without pulling the stitch tight. Continue stitching back and forth about every 1/4", being careful to keep your needle from punching through the front surface of the leather and to not pull things too tight, which will result in a puckered seam. Tie things off when you've completed your repair, and replace the cover on your cushion- you're done and you've given your sofa extra life!