Solid state disks use NAND memory. NAND memory has a limited number of write cycles, or a limited number of times data may be reliably written to the disk. Once the write cycles of a NAND cell have been used up, data can no longer reliably be written or read.
Defragmentation deliberately moves data from one section of the drive and writes it to another section of the drive to make contiguous blocks of data. Every bit of data that gets moved to create this pointless contiguous block is a wasted write cycle for every NAND cell that's touched by the defrag.
What's more, SSDs
intentionally fragment data with a process known as "wear leveling." Wear leveling breaks writes up and puts them on separate cells so no one cell ever receives more strain than another.
Because of the nature of SSDs (electrical vs. mechanical), fragmentation means diddly because every NAND cell can be accessed at the same speed.
Don't defrag. Turn all excess writes off: Background defragging, indexing service, DOS 8.3 names, last accessed timestamps, etc.
For more information on SSDs, I wrote a piece last year that outlines all their peculiarities and benefits:
Read it here.