Everytime I hit "Play" in my minecraft launcher, it will begin to download or update files. Even after it finishes downloading, when I hit play it will download another file. The same problem occurred when using a different computer with the same minecraft account

Here is the Launcher Log:

[23:22:36 FATAL]: Fatal error launching game. Report this to http://bugs.mojang.com please!
java.io.FileNotFoundException: File '/Users/clay/Library/Application Support/minecraft/assets/indexes/legacy.json' does not exist
at org.apache.commons.io.FileUtils.openInputStream(FileUtils.java:299) ~[launcher.jar:1.6.11]
at org.apache.commons.io.FileUtils.readFileToString(FileUtils.java:1711) ~[launcher.jar:1.6.11]
at net.minecraft.launcher.Launcher.cleanupOrphanedAssets(Launcher.java:253) ~[launcher.jar:1.6.11]
at net.minecraft.launcher.Launcher.performCleanups(Launcher.java:495) ~[launcher.jar:1.6.11]
at net.minecraft.launcher.game.MinecraftGameRunner.launchGame(MinecraftGameRunner.java:221) ~[launcher.jar:1.6.11]
at com.mojang.launcher.game.runner.AbstractGameRunner.onDownloadJobFinished(AbstractGameRunner.java:186) [launcher.jar:1.6.11]
at com.mojang.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:122) [launcher.jar:1.6.11]
at com.mojang.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:12) [launcher.jar:1.6.11]
at com.mojang.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:86) [launcher.jar:1.6.11]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439) [?:1.6.0_65]
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303) [?:1.6.0_65]
at java.util.concurrent.FutureTask.run(FutureTask.java:138) [?:1.6.0_65]
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895) [?:1.6.0_65]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918) [?:1.6.0_65]
at java.lang.Thread.run(Thread.java:695) [?:1.6.0_65]
[23:22:36 ERROR]: java.io.IOException: Stream closed
[23:22:36 ERROR]: Game ended with bad state (exit code 1)
[23:22:36 INFO]: Ignoring visibility rule and showing launcher due to a game crash
[23:22:36 INFO]: Deleting /Users/clay/Library/Application Support/minecraft/versions/1.8.7/1.8.7-natives-1434165755922588000

What I expected to happen was...:
Open the minecraft application.

What actually happened was...:
The launcher downloaded files over and over again.

Steps to Reproduce:
1. Open minecraft launcher
2. Hit "Play"
3. Launcher downloads files
4. When download finishes, hit "Play" again
5. Launcher downloads more files.