The B737 actual speeds, pitch angle, and vertical speeds will vary based on the thrust used for takeoff, temperature, aircraft weight and any other issues that will affect the climb performance/profile.
However, your question indicates you are primarily interested in: "Why does the climb angle of 737-800 peak shortly after take-off before reducing again?"
I can explain the general takeoff and initial climb profile that would apply to similar air carrier type aircraft, (e.g. Boeing, Airbus, etc).
When making the initial rotation for liftoff from the runway, the pilot will rotate to a pitch attitude of only about 8-10 degrees so as not to incur a tail-strike. Once the aircraft is airborne, the pilot will increase the pitch attitude to about 15 degrees. At about 1000 feet above ground level the pilot will decrease the pitch attitude so the aircraft can accelerate. While accelerating, the takeoff-flaps will be retracted using the appropriate speed schedule. Once the flaps are fully retracted, the pilot will increase the pitch attitude and maintain a speed (in the U.S.) not to exceed 250 knots until reaching 10,000 feet above mean sea level. Once above 10,000 feet the airplane will be accelerated to it's normal climb speed, around 280 knots.
The information above illustrates a general profile to explain why during a normal takeoff and initial climb out the pitch attitude (and "climb angle" per your question) changes. Some operators will have standard operating procedures that will differ somewhat but should essentially follow the same basic profile.
I've found a link to a website that appears to have a reasonable explanation specifically of a B737-800 takeoff procedure that may help you. Here is the link entitled Boeing 737-800 Takeoff Procedure (simplified).