Why does my bike get flat all the time?

I have a road bike which can take 700ccX23-26 tires and I have been using this bike for about 2 years and almost did 6000 miles on it. Lately it gets flat all the time. I checked the tire and there does not seem to be any stone stuck in there.

Can anyone help me what could be the other reason. I am so tired of changing the tubes! I changed tires also 3 times ....
the only tube that lasted any longer is the the slime tubes (green ones) ... but they also are heavier !
 
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