Two studies show it was a good decision:

Compared with women who did not receive hormonal therapy, women who received adjuvant tamoxifen for at least five years were 60% less likely to develop an ER (estrogen responsive)-positive second breast cancer but four times more likely to develop an ER-negative second breast cancer.

and

The researchers concluded that lifestyle factors such as obesity, smoking, and drinking could significantly increase the risk of developing a second cancer.

I don't drink or smoke, but I certainly have obesity issues.  Looks like overall, I had a 50% chance of it coming on the left side, too.

FYI:

The results indicated that women who were considered obese (body mass index [BMI] over 30 kg/m2) were 50% more likely to develop a second breast cancer than women who had a BMI lower than 25 kg/m2. Furthermore, women who consumed more than seven drinks per week after their first breast cancer diagnosis had a 70% higher risk of developing a second breast cancer compared with non-drinkers. Finally, women who smoked were more than twice as likely to develop a second breast cancer compared with nonsmokers.

http://www.cancerconsultants.com/lifestyle-factors-increase-risk-second-breast-cancer/