It really depends on the IO you get. Some people have problems visiting "frequently," others do not. I personally have only been over 3 times in the last 3 1/2 years to visit my fiancee (he's made 4 trips to the US to see me, and will be on his 5th when we get married next month). My trips lasted 3 weeks, 3 1/2 months, and 3 weeks, respectively. His lasted 3 months, 3 weeks, 3 months, and 2 weeks. I never had real trouble getting in. I did get questioned on having a boyfriend, which always spawned further questioning, but it was never anything really scary--I always got in with pretty much no problems. Then again, like I said, I've only made 3 trips, and the longest was for a summer. Plus, I made it very obvious that I had unbreakable ties to the US every time (I was a full time college student, I had an apartment lease, I had a job to return to, and I brought proof of all of it). They only ever asked to see return tickets, how much money I had, and to see my school ID.
If the IO has reason to believe that you'd attempt to settle (and an SO is typically one of their red flags to check out) then you will get some hassle. You just need to be extra careful and be sure that you have documentation proving that you are indeed returning to the US at the end of your visit (like the paystubs and lease you have been taking. A letter from your employer stating your date back to work can be helpful, too. Just make sure none of you luggage [carry on OR checked] has anything like personal diaries or letters, because if they detain you, they may look through your things and will read anything you've written, and if there's even the slightest mention of an SO, they can take things out of context and use them against you as means of saying you don't intend to leave...even if it's a line as simple as "I miss him so much--I can't wait to see him soon!"). You are not making frequent long trips (which almost always causes problems), and what is considered frequent otherwise is totally up to the IO.
You say that they "noted" how many times you have been over? Do you mean they just pointed it out, or that they physically wrote it down somewhere/entered it into a computer?
If you ever want to take the stress out of it, especially if you plan to make regular trips, even if they are rather short, you can always apply for entry clearance. There are several threads on it in the Visa section, and if you have any specific questions on it, garry and VictoriaS are great people to turn to.